x64: Fix nested exception debugging

As of commit a449b88 ("x64: Enable sleeping in fault context") it's now
safe for another thread to enter a fault handler on the same CPU.  Fix
exception guard to reflect that.

This is needed for demand paging where a page fault from another thread
can happen on the same CPU where a thread is sleeping in the page fault
handler.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>