big-endian CI

[jmr]

To prevent big endian build breakages, we need a runner (e.g. ppc64 or s390x) for the CI workflows.

#469 (comment)

Big endian support is esoteric and low priority, so those wanting a working big endian build will have to provide this. @barracuda156

[jmr]

It looks like the options for running the tests are a self-hosted runner (not sure who's going to pay for this) or qemu (slow).

Maybe there's a way to cross-compile for one of these targets so we can just check it builds.

[barracuda156]

@jmr Cross-compiling is definitely doable, and I think verifying just the build is fine. Anyone using it on BE will report issues, if such arise.