travis: add an s390x job

Travis now supports IBM Z and IBM Power architectures, so let's enable
them in our CI as well.

As libbpf won't compile on ppc64le right now (with current CFLAGS), let
skip it until the issue is resolved, see discussion in
https://github.com/libbpf/libbpf/pull/98#issuecomment-553873098

See: https://blog.travis-ci.com/2019-11-12-multi-cpu-architecture-ibm-power-ibm-z

.travis.yml

@@ -139,6 +139,14 @@ jobs:
               - sudo $CI_MANAGERS/xenial.sh
               - set +e
 
+        - name: Ubuntu Xenial (s390x)
+          arch: s390x
+          language: bash
+          script:
+              - set -e
+              - sudo $CI_MANAGERS/xenial.sh
+              - set +e
+
         - stage: Coverity
           language: bash
           env: