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/debug.h

@@ -0,0 +1,19 @@
+#pragma once
+/// \file debug.h
+/// Debugging utilities
+
+#include "kutil/memory.h"
+
+extern "C" {
+	addr_t get_rsp();
+	addr_t get_rip();
+	addr_t get_frame(int frame);
+
+}
+
+void print_regs(const cpu_state &regs);
+void print_stack(const cpu_state &regs);
+void print_stacktrace(int skip = 0);
+
+#define print_reg(name, value) cons->printf("         %s: %016lx\n", name, (value));
+