KVM: selftests: Add processor code for s390x

Code that takes care of basic CPU setup, page table walking, etc.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190523164309.13345-7-thuth@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>