[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/crt/crt0.s

@@ -0,0 +1,17 @@
+extern main
+extern exit
+extern __init_libj6
+
+global _start:function (_start.end - _start)
+_start:
+	mov rbp, rsp
+	mov rdi, rsp
+	call __init_libj6
+
+	pop rdi
+	mov rsi, rsp
+	call main
+
+	mov rdi, rax
+	call exit
+.end:

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;