linkage_checker: Whitelist another harmless dylib

[woodruffw]

libgcc_s.1.dylib is a variation on libgcc_s_ppc64.1.dylib.

See #2085, Homebrew/homebrew-core#15787

• 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 tests with your changes locally?

---