working cross-compiled builds

Enable cross-compiled builds for multiple platforms. The process is:

* Clone the repo on the specified version flag or BUILDBRANCH,
  defaulting to the latest tag on BUILDBRANCH
* For each specified platform:
  - Check if that build has already happened. If so, move on
  - Build
  - Upload
  - Cut release

If any of the individual platform builds fail, an error is logged, but
the build process continues for other platforms.

A build can be forced on a custom version tag by using the
`-v <version>` flag.