Pause syscall and int 0xee interrupt syscalls

The syscall/sysret instructions don't swap stacks. This was bad but
passable until syscalls caused the scheduler to run, and scheduling a
task that paused due to interrupt.

Adding a new (hopefully temporary) syscall interrupt `int 0xee` to allow
me to test syscalls without stack issues before I tackle the
syscall/sysret issue.

Also implemented a basic `pause` syscall that causes the calling process
to become unready. Because nothing can wake a process yet, it never
returns.

src/drivers/nulldrv/main.s

@@ -1,11 +1,22 @@
 global _start
 _start:
 	xor rbp, rbp	; Sentinel rbp
+	mov r11, 0		; counter
 
 .loop:
-	;mov rax, 1		; DEBUG syscall
-	mov rax, 0		; NOOP syscall
-	syscall
+	mov rax, 1		; DEBUG syscall
+	;mov rax, 0		; NOOP syscall
+	;syscall
+	int 0xee
 
+	inc r11
+	cmp r11, 5
+
+	jle .loop
+
+	mov rax, 3		; PAUSE syscall
+	; syscall
+	int 0xee
+
+	mov r11, 0
 	jmp .loop
-