[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/page_manager.cpp

@@ -1,5 +1,3 @@
-#include <algorithm>
-
 #include "kutil/assert.h"
 #include "kutil/vm_space.h"
 #include "console.h"