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/syscalls.inc

@@ -0,0 +1,12 @@
+SYSCALL(noop,     0)
+SYSCALL(exit,     1)
+SYSCALL(getpid,   0)
+SYSCALL(fork,     0)
+
+SYSCALL(message,  1)
+
+SYSCALL(pause,    0)
+SYSCALL(sleep,    1)
+
+SYSCALL(send,     2)
+SYSCALL(receive,  2)