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();

src/drivers/nulldrv/main.s

@@ -11,7 +11,7 @@ getpid:
 	push rbp
 	mov rbp, rsp
 
-	mov rax, 5  ; getpid syscall
+	mov rax, 3  ; getpid syscall
 	syscall     ; pid is now already in rax, so just return
 
 	pop rbp
@@ -22,7 +22,7 @@ debug:
 	push rbp
 	mov rbp, rsp
 
-	mov rax, 1  ; debug syscall
+	mov rax, 0  ; debug syscall
 	syscall
 
 	pop rbp
@@ -33,7 +33,7 @@ sleep:
 	push rbp
 	mov rbp, rsp
 
-	mov rax, 4  ; sleep syscall
+	mov rax, 6  ; sleep syscall
 	syscall
 
 	pop rbp
@@ -44,7 +44,8 @@ fork:
 	push rbp
 	mov rbp, rsp
 
-	mov rax, 8
+	mov rax, 0
+
 	syscall    ; pid left in rax
 
 	pop rbp