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: NThomas Huth <thuth@redhat.com>
Signed-off-by: NDavid Hildenbrand <david@redhat.com>
Message-Id: <20171208160207.26494-4-david@redhat.com>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Signed-off-by: NCornelia Huck <cohuck@redhat.com>