Test non x86-64 targets before 1.0.0 release

[jamesmunns]

Currently, all CI tests are x86_64. We should have a policy and testing for other platforms, such as ARM, or non-64 bit x86 targets.

[yoshuawuyts]

It's been a while since this was opened; @jamesmunns is this something you can / would like to pick up?

I believe github actions don't support non-x86-64 targets. What do we do here? Can we close the issue and reconsider later?

[taiki-e]

I'd like to use cross; see this change in #286