[libc] Add stubbed-out stdio and libdl functions

In order to fix link errors with libunwind, stub out these functions for
now.

src/libraries/libc/stdio/fwrite.cpp

@@ -0,0 +1,3 @@
+#include <stdio.h>
+
+size_t fwrite(const void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream) { return 0; }