gh-112529: Make the GC scheduling thread-safe (#114880)

The GC keeps track of the number of allocations (less deallocations)
since the last GC. This buffers the count in thread-local state and uses
atomic operations to modify the per-interpreter count. The thread-local
buffering avoids contention on shared state.

A consequence is that the GC scheduling is not as precise, so
"test_sneaky_frame_object" is skipped because it requires that the GC be
run exactly after allocating a frame object.

Include/internal/pycore_tstate.h

@@ -28,6 +28,7 @@ typedef struct _PyThreadStateImpl {
     PyThreadState base;
 
 #ifdef Py_GIL_DISABLED
+    struct _gc_thread_state gc;
     struct _mimalloc_thread_state mimalloc;
     struct _Py_object_freelists freelists;
     struct _brc_thread_state brc;