ci: add ppc64le & s390x manylinux2014 builds

[mayeut]

This adds ppc64le & s390x builds to travis-ci in order to provide manylinux2014 wheels for those platforms.

Fix #125

[jcfr]

@mayeut as the dependent PRs have been merged, I just rebased this one and tweaked the commit message associated with the pinning of OpenSSL.

Do not hesitate to further tweak.

[mayeut]

@jcfr,

I think we did approximately the same thing at the same time on this one. Thus I discarded your changes after rebasing on master by mistake & can't see the tweaks on OpenSSL anymore...

I'm done for the day, shouldn't conflict anymore on force push.

[jcfr]

TravisCI build associated with ppc64le arch failed with a download error related to rust package

Suspecting an intermittent download failure, I restarted the specific travisCI job

[jcfr]

It seems to always fail:

See https://travis-ci.com/github/scikit-build/cmake-python-distributions/jobs/497242977

Relevant discussion, see rust-lang/rls#641 (comment)

[mayeut]

@jcfr, the OpenSSL comment was a leftover of #128 attempts. I just removed it now that #128 has been merged in using new dockcross images (dockcross/dockcross#481) not requiring to pin cryptography to an even lower version.

I updated the arguments of the rust compiler installation too the same ones used in cryptography and that seems to do the trick for the ppc64le download (uses a minimal, stable toolchain).

Finally, there were some resource issues on ppc64le that were worked around limiting the number of parallel builds to 4.

[henryiii]

@jcfr Looks good to me. Worth including before 3.19.1, perhaps?

[henryiii]

To be useful, this probably needs to be applied to https://github.com/scikit-build/ninja-python-distributions too?