Make CI cross-compile lf for every supported platform; support darwin-arm64

[ilyagr]

This reworks the gen/xbuild.sh script to exit with a failure if any of the targets don't compile. It also adds a CI job to try cross-compiling for every platform on every pull request.

The new check is a little slow and compute-expensive, but I am hoping it's well within the quota.

See #1299 for a demo of how the CI would have failed after #1288 and before #1295.

Example of a specific CI failure for #1299:

---

Also added support for darwin-arm64, as that has at least one user and it seems to work fine.

[joelim-work]

Thanks for working on this, apart from #1288, this would have also caught the df errors introduced in #1168 that broke the build for a few platforms when releasing r29.

BTW I happened to be playing around with this myself, you can have a look at my version and see if you want to salvage from there.

[ilyagr]

Thank you for looking this over! It's past my bedtime; I'll look at your version and your other comments at some later point.

🌃💤

Update: I did glance at your version. I like the function you call repeatedly much better than my for loop.

[joelim-work]

BTW I forgot to mention but you may also want to run this through shellcheck. I usually use it to see if there's any variables I forgot to quote, or other mistakes that are easy to make when writing shell scripts.

[ilyagr]

OK, I updated this and borrowed quite a few of your ideas. Thank you very much!

I am also a fan of shellcheck. It demanded a bit too much quoting for my taste, but it's probably still a good idea to make scripts shellcheck-clean.

[ilyagr]

I added one more commit making the script a little more complex but decreasing repetition. Let me know if I should get rid of it.

[joelim-work]

I think the changes are OK, but it's probably best to have @gokcehan look at this as a secondary reviewer. Shell scripts are notorious for being error prone and subject to stylistic preferences.

Thanks once again for the changes!

[gokcehan]

Thanks @ilyagr for the patch and @joelim-work for the review. gen/xbuild.sh looks a lot cleaner now.

I wasn't sure about the additional computation that comes with cross compiling but I guess this is getting necessary these days. I found the quotes for github actions here and it says:

GitHub Actions usage is free for standard GitHub-hosted runners in public repositories, and for self-hosted runners. For private repositories, each GitHub account receives a certain amount of free minutes and storage for use with GitHub-hosted runners, depending on the product used with the account. Any usage beyond the included amounts is controlled by spending limits.

So hopefully it wouldn't be a problem since this is a public repo. The only disadvantage might be the additional delay for each change but I think that is acceptable.