plan9: use _Exit

Author: Noam Preil

src/core/os.c

@@ -282,11 +282,7 @@ JANET_CORE_FN(os_exit,
     }
     janet_deinit();
     if (argc >= 2 && janet_truthy(argv[1])) {
-#ifdef JANET_PLAN9
-		exit(status);
-#else
         _Exit(status);
-#endif
     } else {
         exit(status);
     }