CI: test on GOARCH=arm64

[mvdan]

arm64 would be a good candidate, given how increasingly popular it's becoming on laptops and portable devices.

This means running go test ./... on GOARCH=arm64; it would act as the host architecture, and the tests would target the same architecture too.

For instance, I suspect testdata/scripts/asm.txt will fail, as it currently seems to assume amd64.

I'm not sure what would be the easiest way to do this right now. Perhaps qemu on linux.

[lu4p]

Azure pipelines is free for OSS, supports arm64 and integrates with github well.

https://azure.microsoft.com/en-us/services/devops/pipelines/

[mvdan]

Thanks for the pointer; perhaps that would be best, to avoid the complexity or overhead of qemu emulation. Happy to review a patch if someone wants to take a crack; I'm not particularly familiar with azure CI.

[mvdan]

#426 made CI test on GOARCH=386; repurposing this to be about testing on arm64.