Bug 1466689 - Perform macOS builds on gecko-{L}-b-linux worker type; r=dustin,mshal

The gecko-{L}-b-macosx64 worker types are really Linux (macOS builds are
cross-compiled). These worker types are essentially identical to their
gecko-{L}-b-linux counterparts.

I don't see a compelling reason to maintain separate worker types for
these builds other than maybe cost accounting (worker types are tagged
in AWS land and these tags can be more easily broken out for billing
analysis). But I don't think any important systems are relying on
this "feature."

So let's move the macOS build tasks to the gecko-{L}-b-linux workers.

MozReview-Commit-ID: 67bArn6IG9T