configure: s390x supports mttcg now

s390x is ready. Most likely we are missing some pieces, but it should
already be in pretty good shape now.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180129125623.21729-16-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>