powerpc: Make ARCH=powerpc the default for 32-bit ppc

This makes ARCH=powerpc the default on 32-bit powerpc machines,
where uname -m returns ppc, as well as on 64-bit powerpc machines.
Most people who would be likely to build their own kernels on
32-bit powerpc machines would be using powermacs or CHRP machines,
both of which are supported with ARCH=powerpc now.  Embedded ppc
developers whose ports haven't been moved over to arch/powerpc
yet will have to explicitly set ARCH=ppc now.

Signed-off-by: Paul Mackerras <paulus@samba.org>