gitlab-ci: revert "gitlab-ci: temporarily disable s390x"

The builder is back.

This reverts commit d89be7ee.

.gitlab-ci.yml

@@ -57,6 +57,15 @@ build-x86:
     - ci-build
     - x86
 
+build-s390x:
+  extends: .build
+  artifacts:
+    name: MR${CI_MERGE_REQUEST_ID}_s390x
+  tags:
+    - docker-alpine
+    - ci-build
+    - s390x
+
 build-ppc64le:
   extends: .build
   artifacts: