Use signal-safe functions in signal handler

According to signal-safety(7), exit(3) and puts(3) are not safe to call
in a signal handler.

Author: Tristan Partin <tristan@neon.tech>
Discussion: https://www.postgresql.org/message-id/flat/CTVDKVZCCVSY.1XQ87UL50KQRD%40gonk

src/bin/pg_test_fsync/pg_test_fsync.c

@@ -602,8 +602,8 @@ signal_cleanup(SIGNAL_ARGS)
 	if (needs_unlink)
 		unlink(filename);
 	/* Finish incomplete line on stdout */
-	puts("");
-	exit(1);
+	write(STDOUT_FILENO, "\n", 1);
+	_exit(1);
 }
 
 #ifdef HAVE_FSYNC_WRITETHROUGH