Bootstrap Go on macOS from source

[zombiezen]

As of b21b123, our Go package depends on a binary release rather than building from source. As indicated in the source installation instructions, Go 1.4 does not support aarch64-apple-darwin so one must bootstrap Go 1.4 on a different supported system (x86_64-unknown-linux for us) and then build up to a version of Go that can cross-compile for aarch64-apple-darwin. Because we can share store objects among our Unix-like systems, we can actually do this inside zb.

Once remote building (256lights/zb#44) and caching (256lights/zb#43) is supported, then we should drop the reliance on the binary release and build entirely from source.