CI: add BigEndian test target

[dhardy]

Old Travis had this config to run tests on an emulated MIPS target. We want a test on a Big Endian platform since we have had some errors with this in the past. Help setting this up would be welcome!

    # This target catches endianness issues
    - rust: stable
      sudo: required
      dist: trusty
      services: docker
      name: "Linux (MIPS, big-endian)"
      env: TARGET=mips-unknown-linux-gnu
      install:
        - sh utils/ci/install.sh
        - source ~/.cargo/env || true

Also see #1073

[bluss]

ndarray tests on mips using this config https://github.com/rust-ndarray/ndarray/blob/3a2040d916dd892a76cf7f219c1510cf8e48f34a/.github/workflows/ci.yml#L36-L61 which you can copy as you want

[dhardy]

Thanks @bluss. BTW you don't need cross for i686-unknown-linux-gnu but in that case you do need gcc-multilib as here.