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.h

@@ -43,6 +43,7 @@ struct process
 	// be sure to change the assembly definitions in 'tasking.inc'
 	uintptr_t rsp;
 	uintptr_t rsp0;
+	uintptr_t rsp3;
 	page_table *pml4;
 	// End of assembly fields