s390x and ppc: Fill in CPU model field

This patch fills in the 'model' field with the most relevant field data
from /proc/cpuinfo on s390x and PPC. Previously, this used to be just 0.

The PPC part of the patch was contributed by:
Michael Petlan (mpetlan@redhat.com)

Bug: 1072444
Change-Id: I23bf0df8cc9f19000bb7657b4b55930eb407d809