[libc] Add new libc

This new libc is mostly from scratch, with *printf() functions provided
by Marco Paland and Eyal Rozenberg's tiny printf library, and malloc and
friends provided by dlmalloc.

src/libraries/libc/arch/amd64/errno.h

@@ -0,0 +1,5 @@
+#pragma once
+/// \file arch/amd64/errno.h
+/// errno implementation for amd64
+
+extern int errno;