s390x/tcg: implement extract-CPU-time facility

It only provides the EXTRACT CPU TIME instruction. We can reuse the stpt
helper, which calculates the CPU timer value.

As the instruction is not privileged, but we don't have a CPU timer
value in case of linux user, we simply reuse cpu_get_host_ticks() to
produce some descending value.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171208160207.26494-13-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
