don't replace powerpc arch with ppc

'powerpc' is a valid arch for the LINUX_KARCH variable, but the build
system unconditionally replaces LIUNUX_KARCH=powerpc with
LINUX_KARCH=ppc.

This change only does the replacement if LINUX_KARCH isn't set. This
allows us to use the powerpc architecture.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

SVN-Revision: 9405