This makes it possible to build kernels for PReP and/or CHRP
with ARCH=ppc by removing the (non-building) powermac support.
It's now also possible to select PReP and CHRP independently.
Powermac users should now build with ARCH=powerpc instead of
ARCH=ppc.  (This does mean that it is no longer possible to
build a 32-bit kernel for a G5.)
Signed-off-by: Paul Mackerras <paulus@samba.org>