[3.13] pythongh-126914: Store the Preallocated Thread State's Pointer…

… in a PyInterpreterState Field (pythongh-127114)

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

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