spec: Only build x86_64 and ppc64 architectures

We're not supporting any other architectures than x86_64 and ppc64
at the moment. This patch restricts the rpm builds to those
architectures.

Change-Id: Iab30dd4eba6fd068e9605312bfc1c90b03b01564
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32125
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Michal Skrivanek <michal.skrivanek@redhat.com>