__powerpc__ is not defined on AIX and Darwin, Makefile.target adds
it to CPPFLAGS for target specific code which cache-utils are not.

Since there's not common definition which can safely be used on all
platforms use HOST_PPC provided by config-host.h

The problem was originally reported by C. W. Betts

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6292 c046a42c-6fe2-441c-8c8c-71466251a162