[POWERPC] Provide access to arch/powerpc include path on ppc64

There does not appear to be any reason that we shouldn't just have
-Iarch/$(ARCH) on both ppc32 and ppc64 builds.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>