[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/include/kernel_memory.h

@@ -17,7 +17,7 @@ namespace memory {
 	constexpr uintptr_t page_offset = 0xffffc00000000000ull;
 
 	/// Max number of pages for a kernel stack
-	constexpr unsigned kernel_stack_pages = 4;
+	constexpr unsigned kernel_stack_pages = 2;
 
 	/// Max number of pages for a kernel buffer
 	constexpr unsigned kernel_buffer_pages = 16;