gh-116522: Stop the world before fork() and during shutdown (#116607)

This changes the free-threaded build to perform a stop-the-world pause
before deleting other thread states when forking and during shutdown.
This fixes some crashes when using multiprocessing and during shutdown
when running with `PYTHON_GIL=0`.

This also changes `PyOS_BeforeFork` to acquire the runtime lock
(i.e., `HEAD_LOCK(&_PyRuntime)`) before forking to ensure that data
protected by the runtime lock (and not just the GIL or stop-the-world)
is in a consistent state before forking.

Python/pylifecycle.c

@@ -1911,6 +1911,9 @@ Py_FinalizeEx(void)
     int malloc_stats = tstate->interp->config.malloc_stats;
 #endif
 
+    /* Ensure that remaining threads are detached */
+    _PyEval_StopTheWorldAll(runtime);
+
     /* Remaining daemon threads will automatically exit
        when they attempt to take the GIL (ex: PyEval_RestoreThread()). */
     _PyInterpreterState_SetFinalizing(tstate->interp, tstate);