Testing big endian code with Travis CI's new s390x support

Hi,

This is something I've wanted several times in the past, so I thought
others here could be interested:  if you're looking for a way to run
your development branch through check-world on a big endian box, the
new s390x support[1] on Travis is good for that.  Capacity is a bit
limited, so I don't think I'll point cfbot.cputube.org at it just yet
(maybe I need to invent a separate slow build cycle).

I tried it just now and found that cfbot's .travis.yml file[2] just
needed this at the top:

arch:
  - s390x

... and then it needed these lines commented out:

#before_install:
#  - echo '/tmp/%e-%s-%p.core' | sudo tee /proc/sys/kernel/core_pattern

I didn't look into why, but otherwise that fails with a permission
error on that environment, so it'd be nice to figure out what's up
with that so we can still get back traces from cores.

[1] https://blog.travis-ci.com/2019-11-12-multi-cpu-architecture-ibm-power-ibm-z
[2] https://github.com/macdice/cfbot/blob/master/travis/.travis.yml