Add new "cross" bundle to cross-compile the Docker client

This PR depends on #3275 (hopefully for very obvious reasons).

We currently only do darwin/386 and darwin/amd64, but I hope to add linux/386 and linux/arm in the near future (especially since it'd be mostly as easy as converting our _linux and _darwin files to be _supported and _unsupported with nice os/arch build tags), and then look to adding windows/386 and windows/amd64 and eventually get the BSDs going on too.

This is much better than my previous PR (#2010) that did something similar because it preserves our Dockerfile cache by enumerating our supported cross-platforms in an ENV in the Dockerfile directly and always working from that variable when we need to do something with the individual platforms.