[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/arch/x86_64/_Exit.s

@@ -0,0 +1,5 @@
+global _PDCLIB_Exit
+_PDCLIB_Exit:
+	; arg should already be in rdi
+	mov rax, 0x11 ; Exit syscall
+	syscall