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.

Lib/test/test_gc.py

@@ -363,6 +363,7 @@ class GCTests(unittest.TestCase):
     # To minimize variations, though, we first store the get_count() results
     # and check them at the end.
     @refcount_test
+    @unittest.skipIf(Py_GIL_DISABLED, 'needs precise allocation counts')
     def test_get_count(self):
         gc.collect()
         a, b, c = gc.get_count()