gh-99113: Share the GIL via PyInterpreterState.ceval.gil (gh-104203)

In preparation for a per-interpreter GIL, we add PyInterpreterState.ceval.gil, set it to the shared GIL for each interpreter, and use that rather than using _PyRuntime.ceval.gil directly.  Note that _PyRuntime.ceval.gil is still the actual GIL.