[3.13] gh-126914: Store the Preallocated Thread State's Pointer in a …

…PyInterpreterState Field (gh-127114)

This approach eliminates the originally reported race.  It also gets rid of the deadlock reported in gh-96071, so we can remove the workaround added then.

This is mostly a cherry-pick of 1c0a104 (AKA gh-126989).  The difference is we add PyInterpreterState.threads_preallocated at the end of PyInterpreterState, instead of adding PyInterpreterState.threads.preallocated.  That avoids ABI disruption.