Fix Bazel rules on OS X for Go
Fix Bazel rules on OS X for Go

Summary:
We need to detect darwin in a different way.

Some C++ tests are still upset, but it would be better
to land the LLVM update first.

Reviewers: schroederc, fromberger, shahms

Reviewed By: fromberger, shahms

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D567

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