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/syscall.s

@@ -1,53 +1,52 @@
 %include "push_all.inc"
+%include "tasking.inc"
+
+%define SYSCALL(name, nargs)     resb 1
+
+struc SYSCALLS
+%include "syscalls.inc"
+.count:
+endstruc
+
 
 extern __counter_syscall_enter
 extern __counter_syscall_sysret
 
-extern syscall_handler
-extern isr_handler_return
+extern syscall_registry
+extern syscall_invalid
+
 global syscall_handler_prelude
 syscall_handler_prelude:
 	swapgs
-	mov [gs:0x08], rsp
-	mov rsp, [gs:0x00]
+	mov [gs:CPU_DATA.rsp3], rsp
+	mov rsp, [gs:CPU_DATA.rsp0]
 
-	push 0x23   ; ss
-	push 0x00   ; rsp - to be filled
-	push r11    ; rflags
-	push 0x2b   ; cs
-	push rcx    ; user rip
-	push 0      ; bogus error
-	push 0      ; bogus vector
-	push_all
+	push rcx
+	push rbp
+	mov rbp, rsp
+	push r11
 
 	inc qword [rel __counter_syscall_enter]
 
-	mov rax, [gs:0x08]
-	mov [rsp + 0xa0], rax
-	mov rax, [rsp + 0x70]
+	cmp rax, SYSCALLS.count
+	jl .ok_syscall
 
-	mov rdi, rsp
-	call syscall_handler
+	mov rdi, rax
+	call syscall_invalid
 
-	mov rax, [rsp + 0x90]
-	and rax, 0x3
-	cmp rax, 0x3
-	jne isr_handler_return
+.ok_syscall:
+	lea r11, [rel syscall_registry]
+	mov r11, [r11 + rax * 8]
+	call r11
 
 	inc qword [rel __counter_syscall_sysret]
 
-	mov rax, [rsp + 0xa0]
-	mov [gs:0x08], rax
+	pop r11
+	pop rbp
+	pop rcx
 
-	pop_all
-	add rsp, 16 ; ignore bogus interrupt / error
-	pop rcx     ; user rip
-	add rsp, 8  ; ignore cs
-	pop r11     ; flags
-	add rsp, 16 ; rsp, ss
-
-	mov [gs:0x00], rsp
-	mov rsp, [gs:0x08]
+	mov [gs:CPU_DATA.rsp0], rsp
+	mov rsp, [gs:CPU_DATA.rsp3]
 
 	swapgs
 	o64 sysret