x86: 32-bit: allow direct ISRs only with !KPTI

This doesn't work as expected with kernel page table isolation
turned on, and fixing it would likely lose any latency benefits
that direct ISRs are supposed to provide.

For now, just prevent these macros from being defined if KPTI
is turned on, like other arches that do not implement this
feature.

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