Add rustc flags

[carterbox]

Adds the appropriate target-cpu argument for rustc.

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [ ] Ensured the license file is being packaged.

https://rust-lang.github.io/packed_simd/perf-guide/target-feature/rustflags.html

My understanding is the the target-cpu optimization for rust is whatever is available from LLVM. I don't think LLVM offers optimizations for powerpc, or maybe I just don't recognize the name of target that would be used.

[conda-forge-webservices[bot]]

Hi! This is the friendly automated conda-forge-linting service.

I wanted to let you know that I linted all conda-recipes in your PR (recipe) and found some lint.

Here's what I've got...

For recipe:

• The recipe must have some tests.