GOARM propose PR

[franciscocpg]

Hi

I'd like to propose a PR for working with GOARM conforming to https://github.com/golang/go/wiki/GoArm.

Because it says:

Starting from Go 1.1, the appropriate GOARM value will be chosen if you compile the program from source on the target machine. In cross compilation situations, it is recommended that you always set an appropriate GOARM value along with GOARCH.

Well, as gox deals with cross compilation it needs to deal with GOARM.

If you have interest on this please let me know and then I am going to start working on this.