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

[ericsnowcurrently]

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 1c0a104eca189a932e0b44ca9bef46cce3d0b801 (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.

• Issue: gh-126914

[ericsnowcurrently]

@Yhg1s, this changes only internal ABI, so we should be fine updating the ABI data file, right?

[Yhg1s]

Yes, the struct is always allocated by the runtime, and the new field is used only by the runtime, never extension modules, so updating the ABI definition is fine.