Posix: add '#pragma GCC poison exit'

Generally exit() should not be used on Posix, because it exits the whole
program instead of only the task.

src/platforms/apps.cpp.in

@@ -48,7 +48,7 @@ void list_builtins(apps_map_type &apps)
 int shutdown_main(int argc, char *argv[])
 {
 	printf("Shutting down\n");
-	exit(0);
+	system_exit(0);
 }
 
 int list_tasks_main(int argc, char *argv[])