irq: Introduce CPU_INTERRUPT_TGT_* defines.

These defines will be place-holders for cpu-specific functionality.
Generic code will, at the end of the patch series, no longer have to
concern itself about how SMI, NMI, etc should be handled.  Instead,
generic code will know only that the interrupt is internal or external.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>