gem: attempt to build a darwin gem that works on multiple darwin versions

[flavorjones]

related to #2079

See #2075 and #2079 for context. This branch attempts to generate a x86_64-darwin gem, following some of the patterns in the grpc gem but using x86_64 instead of universal.

[AppVeyorBot]

✅ Build nokogiri 1.0.674 completed (commit 75392a921f by @flavorjones)

[AppVeyorBot]

✅ Build nokogiri 1.0.678 completed (commit 3bfeb4a5d2 by @flavorjones)

[codeclimate]

Code Climate has analyzed commit 343e8e0 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 94.3% (0.0% change).

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[larskanis]

@flavorjones Sorry for being so unresponsive in the last months! I was very busy at work. I plan to invest some time into rake-compiler-dock the next week and would like to add the MacOS cross compiler and a darwin target. Did you do anything with this cross compiler? Or everything native on MacOS?

[flavorjones]

@larskanis I was also pretty busy with work and haven't had time to investigate using the cross-compiler. Was planning on queuing that work up for January if you hadn't gotten to it first. (I'm building the darwin images natively on my macbook for now.)