[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/include/errno.h

@@ -0,0 +1,15 @@
+#pragma once
+/* Errors <errno.h>
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include "j6libc/int.h"
+
+#define errno (*_PDCLIB_errno_func())
+
+#define ERANGE _PDCLIB_ERANGE
+#define EDOM _PDCLIB_EDOM
+#define ENOMEM _PDCLIB_ENOMEM
+#define EINVAL _PDCLIB_EINVAL