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/kernel/interrupts.s

@@ -57,6 +57,7 @@ irq_handler_prelude:
 %endmacro
 
 %define EISR(i, name)     EMIT_EISR name, i
+%define UISR(i, name)     EMIT_ISR name, i
 %define  ISR(i, name)     EMIT_ISR name, i
 %define  IRQ(i, q, name)  EMIT_IRQ name, i