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/debug.cpp

@@ -59,7 +59,7 @@ print_stacktrace(int skip)
 	int fi = -skip;
 	__asm__ __volatile__ ( "mov %%rbp, %0" : "=r" (fp) );
 
-	while (fp) {
+	while (fp && fp->return_addr) {
 		if (fi++ >= 0)
 			cons->printf("  frame %2d: %lx\n", fi-1, fp->return_addr);
 		fp = fp->prev;