includes one of bus_at386.h and bus_pc98.h. Becuase only bus_pc98.h
supports indirect pio and bus_at386.h is identical to old bus.h, there
is no functional change in PC-AT's kernels. That is, it cannot cause
performance loss.
Submitted by: nyan
Reviewed by: imp
bde and luoqi provided useful comments for earlier version.
