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.cpp

@@ -0,0 +1,60 @@
+#include "console.h"
+#include "cpu.h"
+#include "debug.h"
+#include "scheduler.h"
+#include "syscall.h"
+
+extern "C" {
+	void _halt();
+}
+
+addr_t
+syscall_dispatch(addr_t return_rsp, const cpu_state &regs)
+{
+	console *cons = console::get();
+	syscall call = static_cast<syscall>(regs.rax);
+
+	switch (call) {
+	case syscall::noop:
+		break;
+
+	case syscall::debug:
+		cons->set_color(11);
+		cons->printf("\nReceived DEBUG syscall\n");
+		cons->set_color();
+		print_regs(regs);
+		break;
+
+	case syscall::message:
+		cons->set_color(11);
+		cons->printf("\nReceived MESSAGE syscall\n");
+		cons->set_color();
+		break;
+
+	case syscall::pause:
+		{
+			cons->set_color(11);
+
+			auto &s = scheduler::get();
+			auto *p = s.current();
+			p->flags -= process_flags::ready;
+			//log::debug(logs::task, "Pausing process %d, flags: %08x", p->pid, p->flags);
+			cons->printf("\nReceived PAUSE syscall\n");
+			return_rsp = s.tick(return_rsp);
+			//log::debug(logs::task, "Switching to stack %016lx", return_rsp);
+			cons->printf("\nDONE WITH PAUSE syscall\n");
+			cons->set_color();
+		}
+		break;
+
+	default:
+		cons->set_color(9);
+		cons->printf("\nReceived unknown syscall: %02x\n", call);
+		cons->set_color();
+		_halt();
+		break;
+	}
+
+	return return_rsp;
+}
+