[libc] Implement sbrk to allow malloc() to work

Userspace can now allocte via malloc. This is slightly janky because it
relies on a single static handle in the library code.

src/libraries/libc/include/j6libc/syscalls.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <j6/types.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif