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.