gh-117657: Fix race involving immortalizing objects (#119927)

The free-threaded build currently immortalizes objects that use deferred
reference counting (see gh-117783). This typically happens once the
first non-main thread is created, but the behavior can be suppressed for
tests, in subinterpreters, or during a compile() call.

This fixes a race condition involving the tracking of whether the
behavior is suppressed.

Python/pystate.c

@@ -1583,9 +1583,7 @@ new_threadstate(PyInterpreterState *interp, int whence)
     }
     else {
 #ifdef Py_GIL_DISABLED
-        if (interp->gc.immortalize.enable_on_thread_created &&
-            !interp->gc.immortalize.enabled)
-        {
+        if (_Py_atomic_load_int(&interp->gc.immortalize) == 0) {
             // Immortalize objects marked as using deferred reference counting
             // the first time a non-main thread is created.
             _PyGC_ImmortalizeDeferredObjects(interp);