[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/seek.c

@@ -0,0 +1,18 @@
+/* int64_t _PDCLIB_seek( FILE *, int64_t, int )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+/* This is an example implementation of _PDCLIB_seek() fit for use with POSIX
+   kernels.
+ */
+
+#include <stdio.h>
+#include "j6libc/glue.h"
+
+int64_t _PDCLIB_seek( struct _PDCLIB_file_t * stream, int64_t offset, int whence )
+{
+	_PDCLIB_errno = _PDCLIB_ERROR;
+    return EOF;
+}