x86: close EOI race condition when nested IRQs enabled

Until now, EOI had always been sent out to the APIC with interrupts
unlocked. Depending on timing, there is a race where the next interrupt
on the same line could arrive before _IntExit disables interrupts
and pops context. If this happens consecutively enough times, the
interrupt stack will overflow.

Now we disable interrupts at the beginning of _IntExitWithEoi and they
remain that way until 'iret'.

Change-Id: Ibb28e0db902ff483d7a885389f231ac2d1864657
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>