[kernel] Use IST for kernel stacks for NMI, #DF, #PF

We started actually running up against the page boundary for kernel
stacks and thus double-faulting on page faults from kernel space. So I
finally added IST stacks. Note that we currently just
increment/decrement the IST entry by a page when we enter the handler to
avoid clobbering on re-entry, but this means:

* these handlers need to be able to operate with only a page of stack
* kernel stacks always have to be >1 pages
* the amount of nesting possible is tied to the kernel stack size.

These seem fine for now, but we should maybe find a way to use something
besides g_kernel_stacks to set up the IST stacks if/when this becomes an
issue.

src/kernel/interrupts.s

@@ -53,9 +53,8 @@ isr_handler_return:
 		jmp irq_handler_prelude
 %endmacro
 
-%define EISR(i, name)     EMIT_EISR name, i   ; ISR with error code
-%define UISR(i, name)     EMIT_ISR name, i    ; ISR callable from user space
-%define  ISR(i, name)     EMIT_ISR name, i
+%define EISR(i, s, name)     EMIT_EISR name, i   ; ISR with error code
+%define  ISR(i, s, name)     EMIT_ISR name, i
 %define  IRQ(i, q, name)  EMIT_IRQ name, i
 
 section .isrs