Move linkage_checker from os/mac to generic

[sjackman]

• 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 tests with your changes locally?

---

Move check_linkage from os/mac/formula_cellar_checks to generic.

[sjackman]

I'm working on fixing the test failures.

[sjackman]

Done.

[sjackman]

Thanks for merging, Alyssa!