rts: Do not call exit() from SIGINT handler

Previously shutdown_handler would call stg_exit if the scheduler was already 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 (closed).