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/drivers/nulldrv/main.cpp

@@ -16,7 +16,7 @@ main(int argc, const char **argv)
 {
 	int32_t pid = getpid();
 	//int32_t child = fork();
-	debug();
+	//debug();
 	for (int i = 1; i < 5; ++i)
 		sleep(i*10);
 	debug();