gh-116622: Android sysconfig updates

[mhsmith]

This is the final feature PR for adding Android support. All that remains after this is test infrastructure and documentation updates.

• Implemented sysconfig.get_platform in the format specified by PEP 738.

• Added the -Wl,--no-undefined linker flag, both for CPython itself, and for third-party packages built using the sysconfig LDFLAGS. Based on past experience of building packages for Android, this can save a lot of time by detecting problems at compile time which would otherwise happen at runtime.

• Added Android to the Tier 3 support list.

• Issue: Add support for Android as a target platform #116622

[mhsmith]

@encukou: Thanks for your help on my Android PRs in the last few days. Could I trouble you to review this one as well? It's pretty small, and it's the final one that needs to get merged before the feature freeze on May 7.

[encukou]

I see this is in the PEP (sorry I didn't catch it) but let me ask -- more to bring this to your attention than anything else:
Is there a specific reason to use Android arch names, rather than ones from configure?

For reference: Fedora used their own arch names, and spent several years migrating to the configure ones when wheels became more universal.
Android wheels will probably always be separate, but it seems they might be compared to other Linux-y wheels more often than other Android apps. Also, maintaining this dict in CPython might not be worth it.

If CPython-style arch names would be better, this is the time to change them.

[mhsmith]

We are using autoconf-style architecture names everywhere else, including in extension module filenames (see test_android_ext_suffix in test_sysconfig.py). This PR really only affects the platform tag in wheel filenames.

The main reason to use this format is for consistency with the existing Chaquopy wheel repository, which contains over a thousand wheels for Python 3.8-3.12. Having two formats in use would complicate package installers such as pip (which we intend to contribute Android and iOS support for in the near future), and higher-level app building tools such as Briefcase.

[encukou]

We are using autoconf-style architecture names everywhere else

Right, and this inconsistency is somewhat jarring.
If you'd be interested in setting up aliases in the Chaquopy repository, or adding aliases to packaging, let me know & I can help.

But, no reason for that to hold this PR back.

[encukou]

Hah, it was the next thing on my list when you wrote that!

Looks good, tell me to merge & I will :)