Currently the developer running on Windows should use set GOOS=linux and then run the gopherjs. Have some issues about it: #875, #776.

This changes makes the gopherjs to use the GOOS=linux, clearly telling that the GOOS was changed, the user can still use the GOOS=darwin or GOOS=linux manually.

The README says:

gopherjs uses your platform's default GOOS value when generating code. Supported GOOS values are: linux, darwin. If you're on a different platform (e.g., Windows or FreeBSD), you'll need to set the GOOS environment variable to a supported value. For example, GOOS=linux gopherjs build [package].

This change makes the gopherjs use the GOOS=linux by default on any unsupported GOOS and plataform. If the user are using darwin it will still darwin, even if it's not set on GOOS explicit.

I only test with the gopherjs build, maybe more test should be done.