.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>

.shippable.yml

@@ -5,6 +5,8 @@ env:
       TARGET_LIST=arm-softmmu,arm-linux-user
     - IMAGE=debian-arm64-cross
       TARGET_LIST=aarch64-softmmu,aarch64-linux-user
+    - IMAGE=debian-s390x-cross
+      TARGET_LIST=s390x-softmmu,s390x-linux-user
 build:
   pre_ci:
     - make docker-image-${IMAGE}