Add support for MIPS architectures in Travis

[mpapierski]

This adds mips/mips64/mipsel/mips64el targets to the build system.

Unfortunately travis trusty image doesn't ship with gcc-mips*
packages. On these platforms cargo is replaced with cross1 to run
those builds on a docker container.

For now all MIPS tests are allowed to fail to make way for future PRs
that will fill the gaps.

This is a first step to bring support to MIPS platforms (#562).

Note that the with this commit matrix really grew up to about 168 builds. This is fine since we have 'fast_finish(no waiting forallow_failures` builds).

I'm still waiting for a full build to finish on my fork (https://travis-ci.org/mpapierski/ring). Note that at first I've disabled few tests just to make way for mips builds, and I plan to re-run cancelled tests that are known to be green just to make sure everything works as expected.

Expected failure on all mips architectures:

--- stderr
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
note: Run with `RUST_BACKTRACE=1` for a backtrace.

[briansmith]

OK, looking at this, it seems like MIPS is a lot more expensive to support than I expected.

Is there really no alternative to using docker and the cross toolkit? I think that on other targets we use sudo: true and then install packages. I would prefer to keep MIPS more like the other platforms if at all possible.

[mpapierski]

@briansmith Instead of running builds through cross it could be probably achieved in a smiliar way by specifying linker to a cross compiling one, and the runner would be qemu-$ARCH. That would work. Anyway, I'd still recommend cross as it saves a lot of headaches. We have lots of successes with it especially when dealing with MIPS and big endian. We even run x86 tests through cross.

[briansmith]

OK, looking at this, it seems like MIPS is a lot more expensive to support than I expected.

Is there really no alternative to using docker and the cross toolkit? I think that on other targets we use sudo: true and then install packages. I would prefer to keep MIPS more like the other platforms if at all possible.

I have decided, at least for now, to use cross for all targets except i686 and x86_64. I've extended your PR to support this on this branch: https://travis-ci.org/briansmith/ring/builds/468249568 and I'll merge it once CI passes.

[pietro]

@mpapierski @briansmith Travis now has a Xenial environment that so we can use it for cross compiling mips. I'll add it to nightly only on a branch in my fork. This way it will only add 8 more builds instead of 24.

[briansmith]

I had to move away from Travis CI to GitHub Actions. Further, I learned a lot about how to do this cross-compilation stuff so that we don't need to use cross. So, this PR is obsolete and I'm going to close it. It isn't your fault; I just didn't get to it in a timely manner. Thanks for contributing this. I will update issue #562 with my current plans.