[3.13] gh-117657: Fix data races when writing / reading `ob_gc_bits` (GH-118292) (#118796)

Use relaxed atomics when reading / writing to the field. There are still a
few places in the GC where we do not use atomics. Those should be safe as
the world is stopped.
(cherry picked from commit cb6f75a32ca2649c6cc1cabb0301eb783efbd55b)

Co-authored-by: mpage <mpage@meta.com>