LibC: Implement __freading and __fwriting

These functions are used by gnulib (and therefore many GNU utilities)
to provide access to internal details of the stdio FILE structure.

Userland/Libraries/LibC/stdio_ext.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <stdio.h>
+
+__BEGIN_DECLS
+
+int __freading(FILE*);
+int __fwriting(FILE*);
+
+__END_DECLS