gh-130030: Fix crash on 32-bit Linux with free threading (gh-130043)

The `gc_get_refs` assertion needs to be after we check the alive and
unreachable bits. Otherwise, `ob_tid` may store the actual thread id
instead of the computed `gc_refs`, which may trigger the assertion if
the `ob_tid` looks like a negative value.

Also fix a few type warnings on 32-bit systems.