Improve syscalls for new task switching

There are a lot of under the hood changes here:
- Move syscalls to be a dispatch table, defined by syscalls.inc
- Don't need a full process state (push_all) in syscalls now
- In push_all, define REGS instead of using offsets
- Save TWO stack pointers as well as current saved stack pointer in TCB:
  - rsp0 is the base of the kernel stack for interrupts
  - rsp3 is the saved user stack from cpu_data
- Update syscall numbers in nulldrv
- Some asm-debugging enhancements to the gdb script
- fork() still not working

src/kernel/process.cpp

@@ -35,11 +35,7 @@ process::fork(cpu_state *regs)
 			kernel_stack, child->kernel_stack, child->rsp);
 
 	child->setup_kernel_stack();
-	task_fork(child); // Both parent and child will return from this
-
-	if (bsp_cpu_data.tcb->pid == child->pid) {
-		return 0;
-	}
+	task_fork(child);
 
 	return child->pid;
 }