[llvm] [libc][workflow] improve ci coverage with windows-2025 and arm ubuntu (PR #123745)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

Seems reasonable enough to me.

We need to be a bit careful with how many jobs run at once, but from what I've seen the libc jobs are reasonably fast, so shouldn't be a big issue. I'm not too sure how the new ARM runners counter against the limits though (their own lower limit? the general linux limit?).

Once the new premerge is spun up, I would like to reevaluate what configurations we're testing here too. We definitely can't test all the new configurations in the monorepo-wide premerge pipeline, but we do want to test a "standard" build on Linux (and windows if reasonable), and it would be good to not duplicate that with what's being done here.

https://github.com/llvm/llvm-project/pull/123745