gh-117657: TSAN fix race on gstate->young.count

[SonicField]

In gc_free_threading.c at line 1016 the gcstate->young.count is batch incremented via an atomic add. However, at 1162 the count is set to zero during garbage collection and that set is not atomic. This causes a race condition with the addition. Changing to an atomic set solves the race.

• Issue: Make TSAN tests pass with the GIL disabled in free-threaded builds #117657

[ghost]

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[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[SonicField]

@DinoV@mpage@colesbury Here is my first go at a pull request. I cannot figure out how to add you are reviewers; I don't think I have permissions to do so or some such.

[colesbury]

Thanks Alex - I left a few comments below

[colesbury]

There are two compiler warnings (see the "Files changed" tab) and the NEWS file can be removed.

[SonicField]

Hopefullt fixed the compiler warnings (my bad).

[colesbury]

Thanks Alex!