[libc] Bring libc in-tree

Moving libc from its separate repo into this one, minor resulting build
fixes, and a hacky way to add -I for libc headers in builds.

src/libraries/libc/j6libc/assert.c

@@ -0,0 +1,19 @@
+/* _PDCLIB_assert( const char * )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+
+#include "j6libc/aux.h"
+
+void _PDCLIB_assert( const char * const message1, const char * const function, const char * const message2 )
+{
+    fputs( message1, stderr );
+    fputs( function, stderr );
+    fputs( message2, stderr );
+    abort();
+}