In a free-threading build, gc.set_debug() stored gcstate->debug with a
plain write while gc.get_debug() and the collector read it without
synchronisation. gc.set_debug() takes no lock and, unlike
gc_set_threshold_impl(), does not run under _PyEval_StopTheWorld(), so
ThreadSanitizer reports a write/write race on gcstate->debug.

This accesses the flag with FT_ATOMIC_STORE_INT_RELAXED /
FT_ATOMIC_LOAD_INT_RELAXED in gc_set_debug_impl() / gc_get_debug_impl()
and in the collector reads in Python/gc_free_threading.c. Four of those
collector reads run outside stop-the-world (in delete_garbage(),
handle_legacy_finalizers(), and two stats reads in gc_collect_main()) and
race the write directly; the read in gc_collect_internal() runs under
stop-the-world and is made atomic only for consistency. Relaxed ordering is
correct for an independent int flag, matching how gcstate->enabled and
sys dlopenflags (gh-151644) are already handled.

Python/gc.c (the default GIL build) is left unchanged: it has no concurrent
access, and FT_ATOMIC_* compiles to a plain load/store there anyway.

Verified with a --with-thread-sanitizer --disable-gil build stressing
concurrent gc.set_debug() / gc.get_debug() plus a thread churning cyclic
garbage:

Before: WARNING: ThreadSanitizer: data race ... Write of size 4 ... gc_set_debug (2 reports)
After:  0 data races

A regression test is added under Lib/test/test_free_threading/.

• Issue: Data race on the GC debug flag (gc.set_debug/get_debug) in free-threading builds #153014