Add golang overload (1.2, 1.2.1, 1.2.2, 1.3, 1.3.1)

Later, I want to make that `GOLANG_CROSSPLATFORMS` list in `*/cross/Dockerfile` automated, but this will do just swell for now.  Thinking it over, I realized it really will be better to support a mountain of Go versions here since they sometimes have minor incompatibilities between them and it's useful for projects to pin on a particular minor version instead of just the major releases.