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.

Modules/_xxsubinterpretersmodule.c

@@ -513,6 +513,7 @@ interp_create(PyObject *self, PyObject *args, PyObject *kwds)
 
     // Create and initialize the new interpreter.
     PyThreadState *save_tstate = _PyThreadState_GET();
+    assert(save_tstate != NULL);
     const PyInterpreterConfig config = isolated
         ? (PyInterpreterConfig)_PyInterpreterConfig_INIT
         : (PyInterpreterConfig)_PyInterpreterConfig_LEGACY_INIT;