x86_64: fix crash on nested interrupts

x86_64 supports 4 levels of interrupt nesting, with
the interrupt stack divided up into sub-stacks for
each nesting level.

Unfortunately, the initial interrupt stack pointer
on the first CPU was not taking into account reserved
space for guard areas, causing a stack overflow exception
when attempting to use the last interrupt nesting level,
as that page had been set up as a stack guard.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>