[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/kernel/scheduler.cpp

@@ -1,3 +1,5 @@
+#include <stddef.h>
+
 #include "apic.h"
 #include "clock.h"
 #include "console.h"