rts: Do not call exit() from SIGINT handler

Previously `shutdown_handler` would call `stg_exit` if the scheduler was
Oalready found to be in `SCHED_INTERRUPTING` state (or higher). However,
`stg_exit` is not signal-safe as it calls `exit` (which calls `atexit`
handlers). The only safe thing to do in this situation is to call
`_exit`, which terminates with minimal cleanup.

Fixes #23417.

rts/posix/Signals.c

@@ -522,7 +522,9 @@ shutdown_handler(int sig STG_UNUSED)
     // extreme prejudice.  So the first ^C tries to exit the program
     // cleanly, and the second one just kills it.
     if (getSchedState() >= SCHED_INTERRUPTING) {
-        stg_exit(EXIT_INTERRUPTED);
+        // N.B. we cannot use stg_exit() here as it calls exit() which is not
+        // signal-safe. See #23417.
+        _exit(EXIT_INTERRUPTED);
     } else {
         interruptStgRts();
     }