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/interrupts.cpp

@@ -20,7 +20,6 @@ extern "C" {
 
 	void isr_handler(cpu_state*);
 	void irq_handler(cpu_state*);
-	void syscall_handler(cpu_state*);
 
 #define ISR(i, name)     extern void name ();
 #define EISR(i, name)    extern void name ();
@@ -219,9 +218,11 @@ isr_handler(cpu_state *regs)
 		_halt();
 		break;
 
+	/*
 	case isr::isrSyscall:
 		syscall_dispatch(regs);
 		break;
+	*/
 
 	case isr::isrSpurious:
 		// No EOI for the spurious interrupt
@@ -284,9 +285,3 @@ irq_handler(cpu_state *regs)
 
 	*reinterpret_cast<uint32_t *>(0xffffff80fee000b0) = 0;
 }
-
-void
-syscall_handler(cpu_state *regs)
-{
-	syscall_dispatch(regs);
-}