Patch from James Cleverdon & John Stultz

Currently the naming for the IO-APIC fields is very confused, we assign
dest_LowestPrio to delivery_mode, and INT_DELIVERY_MODE to dest_mode.
The values are correct, but the naming is wrong - this patch corrects
that confusion. It also moves the definitions of those settings into
subarch, where they belong (we have to use fixed delivery mode for Summit
due to what seems to be an Intel IO-APIC bug with P4 clustered mode).