Use effective_arch for rustflags

[git-mann]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

Fix for issue: #17566

This PR adds a parameter to the rustflags_target_cpu method to provide the desired architecture rather than using a hard-coded value of oldest_cpu. This allows for bottles built locally using Rust to use the native CPU architecture.

I was able to test this locally by interactively installing a bottle and checking the RUSTFLAGS environment variable was set correctly.

[MikeMcQuaid]

This allows for bottles built locally using Rust to use the native CPU architecture.

This means that bottles built locally and bottles built in CI may build different things, breaking both reproducibility and potentially having errors occur in one place and not the other.

[Bo98]

This allows for bottles built locally using Rust to use the native CPU architecture.

This means that bottles built locally and bottles built in CI may build different things, breaking both reproducibility and potentially having errors occur in one place and not the other.

I think they are referring to --build-from-source rather than --build-bottle and on Linux specifically, which is a combo that has always used native (#5505), likely because Homebrew does not have any minimum hardware requirements. Not even any soft-requirements in documentation (with macOS's requirements being consequencal to the minimum OS support so older machines aren't a concern there).

In any case, ENV.effective_arch is what we use for C/C++ so this correctly aligns it with that, and that also supports --bottle-arch.

[MikeMcQuaid]

Ok, thanks @Bo98.

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @git-mann!