s390x/tcg: implement SET CLOCK PROGRAMMABLE FIELD

Needed for machine check handling inside Linux (when restoring registers).

Except for SIGP and machine checks, we don't make use of the register
yet. Sufficient for now.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171208160207.26494-4-david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
