Restore NDKv21 for GitHub Actions

[ppigazzini]

GitHub updated the versions of NDK installed on the Actions runners
breaking the ARM tests.
Restore the NDKv21 using the GitHub suggested mitigation, see:
actions/runner-images#5595

[vondele]

seems like this fix for NDKv21 stopped working. CI currently fails with

/home/runner/work/_temp/0220f0b4-7a24-47d0-8f9b-a2b9745162c3.sh: line 2: aarch64-linux-android21-clang++: command not found

https://github.com/official-stockfish/Stockfish/runs/7594674615?check_suite_focus=true

[ppigazzini]

"21.4.7075529" is still the last r21 NDK:
https://github.com/android/ndk/wiki/Unsupported-Downloads

The GitHUb runner documentation still shows NDK r21:
https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-Readme.md#android

[ppigazzini]

[all OSs] Android ndk-bundle along with old NDK versions will be deprecated on July, 24

actions/runner-images#5879

I'm testing the proposed mitigation.

[vondele]

any luck with this test of the mitigation?

[dav1312]

Should the ARM tests be turned off until the mitigation is applied so not all commits fail?

[vondele]

yes, I would merge such a PR now.

[ppigazzini]

any luck with this test of the mitigation?

No. No other working solutions in the GH thread comments the last time that I have checked.

[vondele]

OK, thanks for testing. Means eventually we need to find a better way to do this, but it is not urgent.

[ppigazzini]

@vondele the problem is that GitHub removed NDK 21 from all the Ubuntu runners. That was the last version with the compiler able to write the TLS segment with the right alignment (with a static build) for qemu. Our workflow didn't show the missing NDK 21 because GitHub set the ANDROID_NDK_HOME variable (used by us to update the PATH) to the last NDK version, so our workflow kept using the compiler from the last NDK 25.

Some references to try to found a solution:
https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2004-Readme.md
termux/termux-packages#8273
https://github.com/Lzhiyong/termux-ndk/blob/master/patches/align_fix.py
https://reviews.llvm.org/D53906

[ppigazzini]

@vondele it turn out that the problem was only the PATH, opened #4267