.shippable: add s390x-cross target

Use the new debian-s390x-cross.docker target to cross compile for
s390.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20170227143028.16428-3-alex.bennee@linaro.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>