gh-126914: Store the Preallocated Thread State's Pointer in a PyInterpreterState Field (gh-126989)

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.

Include/internal/pycore_runtime_init.h

@@ -118,6 +118,9 @@ extern PyTypeObject _PyExc_MemoryError;
     { \
         .id_refcount = -1, \
         ._whence = _PyInterpreterState_WHENCE_NOTSET, \
+        .threads = { \
+            .preallocated = &(INTERP)._initial_thread, \
+        }, \
         .imports = IMPORTS_INIT, \
         .ceval = { \
             .recursion_limit = Py_DEFAULT_RECURSION_LIMIT, \