Run a Travis test on s390x (big endian)

Summary:
Since there is no cmake binary available for this architecture, the
Ubuntu Focal distribution is used which has a cmake 3.16 port for s390x.

Backport of secp256k1 PR696.

Depends on D7610.

Test Plan:
Run the travis build.
https://travis-ci.org/github/Fabcien/secp256k1/builds/731209990

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7630