[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.h

@@ -7,13 +7,11 @@
 /// Enum of all defined ISR/IRQ vectors
 enum class isr : uint8_t
 {
-#define ISR(i, name)     name = i,
-#define EISR(i, name)    name = i,
-#define UISR(i, name)    name = i,
+#define ISR(i, s, name)     name = i,
+#define EISR(i, s, name)    name = i,
 #define IRQ(i, q, name)  name = i,
 #include "interrupt_isrs.inc"
 #undef IRQ
-#undef UISR
 #undef EISR
 #undef ISR