Add rust-std subpackages for iOS development

[oskirby]

I'm working with the Mozilla VPN team and we are making another attempt to migrate our build infrastructure to use Conda for dependency management, and in order to get iOS builds working we would like to add the iOS rust targets to conda-forge.

This pull request should add recipes to download the rust-std-<arch>-apple-ios targets from static.rust-lang.org and install them alongside the rust package.

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [X] Bumped the build number (if the version is unchanged)
• [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)
• [x] ~Ensured the license file is being packaged.~

[conda-forge-webservices[bot]]

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

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[pkgw]

Thanks for submitting this! Looking over the results of a local test build, this PR looks good to me.