Swap libgomp with llvm-openmp

[isuruf]

Checklist

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

See conda-forge/conda-forge.github.io#831

[conda-forge-linter]

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 outputs section contained an unexpected subsection name. run_exports is not a valid subsection name.

[beckermr]

This needs a build number bump.

[beckermr]

@conda-forge-admin, please rerender

[beckermr]

I am confused as to how this swaps the libraries under the hood. The recipes still package libgomp and they don't make any symlinks to help downstream packages out during compilation.

[isuruf]

Forgot to commit changes to build.sh. 😄

[beckermr]

Ahhh great. Are we ok with static builds not being swapped?

https://github.com/conda-forge/gfortran_impl_osx-64-feedstock/pull/11/files#diff-e178b687b10a71a3348107ae3154e44cR119

[isuruf]

Yes, that's fine for now. I don't think there are any packages in conda-forge, that links statically

[beckermr]

LGTM!

I'll wait for you to merge since it was not clear if we should actually merge this (yet) given the other thread.

[isuruf]

Only camb will be affected