Big Endian build on a CI

[mosra]

It should be possible via a Docker VM with the x390x architecture (IBM's mainframe arch, huh) from https://github.com/multiarch/qemu-user-static . But I fail to wrap my head around creating a Docker VM image like that and it's not that much of a priority right now.

TODO:

• Try the approach from https://github.com/varnishcache/varnish-cache/blob/master/.circleci/config.yml
  • Or possibly just the two-line approach from https://stackoverflow.com/a/3339484 is enough?
• Build & run tests there
• Fix BE-specific issues
• Maybe expose this architecture via a CORRADE_TARGET_* macro? (Checklist for adding a new port.)