As Travis CI IBM z pipeine is operational again, I will add s390x again to check
the case. Though Travis s390x infra team is still investigating the root cause.
https://www.traviscistatus.com/

Please revert this commit or comment on the
<https://bugs.ruby-lang.org/issues/20013>, when you see the s390x builds are not
starting again.

Sorry for inconvenience.