Pause syscall and int 0xee interrupt syscalls

The syscall/sysret instructions don't swap stacks. This was bad but
passable until syscalls caused the scheduler to run, and scheduling a
task that paused due to interrupt.

Adding a new (hopefully temporary) syscall interrupt `int 0xee` to allow
me to test syscalls without stack issues before I tackle the
syscall/sysret issue.

Also implemented a basic `pause` syscall that causes the calling process
to become unready. Because nothing can wake a process yet, it never
returns.

src/kernel/syscall.h

@@ -1,10 +1,18 @@
 #pragma once
 
+#include <stdint.h>
+#include "kutil/memory.h"
+
 enum class syscall : uint64_t
 {
 	noop,
 	debug,
 	message,
+	pause,
 
 	last_syscall
 };
+
+struct cpu_state;
+addr_t syscall_dispatch(addr_t, const cpu_state &);
+