[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/stdalign.h

@@ -0,0 +1,12 @@
+#pragma once
+/* Alignment <stdalign.h>
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#define alignas _Alignas
+#define alignof _Alignof
+
+#define __alignas_is_defined 1
+#define __alignof_is_defined 1