Make the garbage collector thread-safe in `--disable-gil` builds

[colesbury]

Feature or enhancement

Python's cyclic garbage collector relies on the global interpreter lock for thread-safety. There are a number of changes needed to make the GC thread-safe in --disable-gil builds.

My intention is to implement these as a series of smaller changes.

• Garbage collection is guarded by gcstate->collecting. This need to be made thread-safe.
• The --disable-gil builds should stop-the-world when finding garbage, but not when calling finalizers or destructors. (depends on Implement stop-the-world functionality (for --disable-gil builds) #111964)
• The --disable-gil builds should find GC-enabled objects by traversing the mimalloc heaps, because the _gc_next/_gc_prev lists are not thread-safe. (Note it's safe to use the lists during stop-the-world pauses; we just can't maintain them safely during normal Python execution).
• The --disable-gil builds should probably use only a single generation to avoid frequent stop-the-world pauses
• Eventually, we can get rid of the GC pre-header in --disable-gil builds to save memory. (This will require updating the trashcan mechanism.)

Remaining work:

• Fix scheduling of free-threaded GC (currently scheduled as if there is a young generation)
• Update Python docs
• Refactor out common code from gc.c and gc_free_threaded.c

See also #111964

Linked PRs

• gh-112529: Use atomic operations for gcstate->collecting #112533
• gh-112529: Track if debug allocator is used as underlying allocator #113747
• gh-112529: Use GC heaps for GC allocations in free-threaded builds #114157
• gh-112529: Implement GC for free-threaded builds #114262
• gh-112529: Remove PyGC_Head from object pre-header in free-threaded build #114564
• gh-112529: Simplify PyObject_GC_IsTracked and PyObject_GC_IsFinalized #114732
• gh-112529: Stop the world around gc.get_referents #114823
• gh-112529: Make the GC scheduling thread-safe #114880
• gh-112529: Use _PyThread_Id() in mimalloc (free-threaded build) #115488
• gh-112529: Revert "Use _PyThread_Id() in mimalloc in free-threaded build" #115524
• gh-112529: Don't untrack tuples or dicts with zero refcount #117370