gh-112075: Add gc shared bits

[DinoV]

This adds the GC shared flags for objects to the gc bit states in free-threaded builds. This will allow marking objects as being shared so that we can use QSBR to free them, enabling lock-free readers for dict (and other) objects.

• Issue: Make dict objects thread-safe in --disable-gil builds #112075

[colesbury]

LGTM!

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86-64 macOS 3.x has failed when building commit bcccf1f.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/366/builds/6539) and take a look at the build logs.
4. Check if the failure is related to this commit (bcccf1f) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/366/builds/6539

Failed tests:

• test.test_multiprocessing_forkserver.test_processes

Summary of the results of the build (if available):

==

Click to see traceback logs

remote: Enumerating objects: 9, done.        
remote: Counting objects:  11% (1/9)        
remote: Counting objects:  22% (2/9)        
remote: Counting objects:  33% (3/9)        
remote: Counting objects:  44% (4/9)        
remote: Counting objects:  55% (5/9)        
remote: Counting objects:  66% (6/9)        
remote: Counting objects:  77% (7/9)        
remote: Counting objects:  88% (8/9)        
remote: Counting objects: 100% (9/9)        
remote: Counting objects: 100% (9/9), done.        
remote: Compressing objects:  20% (1/5)        
remote: Compressing objects:  40% (2/5)        
remote: Compressing objects:  60% (3/5)        
remote: Compressing objects:  80% (4/5)        
remote: Compressing objects: 100% (5/5)        
remote: Compressing objects: 100% (5/5), done.        
remote: Total 5 (delta 4), reused 0 (delta 0), pack-reused 0        
From https://github.com/python/cpython
 * branch                  main       -> FETCH_HEAD
Note: switching to 'bcccf1fb63870c1b7f8abe246e27b7fff343abd7'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:

  git switch -c <new-branch-name>

Or undo this operation with:

  git switch -

Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at bcccf1fb63 gh-112075: Add gc shared bits (#114931)
Switched to and reset branch 'main'

configure: WARNING: pkg-config is missing. Some dependencies may not be detected correctly.

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: libpython3.13d.a(dynamic_annotations.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: libpython3.13d.a(gc_free_threading.o) has no symbols
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: libpython3.13d.a(jit.o) has no symbols
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups
ld: warning: -undefined dynamic_lookup may not work with chained fixups

make: *** [buildbottest] Error 2

[vstinner]

See issue gh-115124: AMD64 Windows11 Bigmem 3.x: test_bigmem failed with !_Py_IsImmortal(FROM_GC(gc)) assertion error. PR #114931 or PR #108038 caused a regression.