LibC: Implement the _Exit function

libstdc++v3 checks whether this function is available and makes
certain functions available in the std namespace if so.

Userland/Libraries/LibC/stdlib.cpp

@@ -1180,3 +1180,8 @@ int unlockpt([[maybe_unused]] int fd)
     return 0;
 }
 }
+
+void _Exit(int status)
+{
+    _exit(status);
+}