of <machine/pc/bios.h> specific to i386 and added a conditional define
for BIOS_PADDRTOVADDR that depends on ISA_HOLE_START. The latter is
undefined on alpha and ia64. Since the former is defined the same on
both alpha and ia64, assume the ISA_HOLE_START dependent definition
is specific to amd64 and use the identity-mapping in all other cases.
This of course is getting uglier every day...