analyzer: Add exit, and _exit replacement, to sm-signal.

Warn about using exit in signal handler and suggest _exit as alternative.

gcc/analyzer/ChangeLog:

	* sm-signal.cc(signal_unsafe_call::emit): Possibly add
	gcc_rich_location note for replacement.
	(signal_unsafe_call::get_replacement_fn): New private function.
	(get_async_signal_unsafe_fns): Add "exit".

gcc/testsuite/ChangeLog:

	* gcc.dg/analyzer/signal-exit.c: New testcase.