gh-132070: Fix PyObject_Realloc thread-safety in free threaded Python

[colesbury]

The PyObject header reference count fields must be initialized using atomic operations because they may be concurrently read by another thread (e.g., from _Py_TryIncref).

• Issue: gh-132070

[colesbury]

Reimplementing the mi_realloc logic in obmalloc.c feels a bit dirty, but it also seems better than the alternatives.

[bedevere-bot]

:robot: New build scheduled with the buildbot fleet by @colesbury for commit 3387cd2415033c0e367d8303c39103594018949b :robot:

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F143441%2Fmerge

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