gox tries to build unavailable OS, arch combinations

[mcandre]

gox often tries to build target tuple combinations that are unavailable, forcing me to run several different custom gox calls instead of a simple gox run.

For example, when I build my ports for go-ios7crypt with gox, I have to separate the gox runs for DragonflyBSD and NaCl from the main GNU libc run, across multiple "crosscompile-.sh" scripts. I understand that musl/Linux is currently unavailable from the official Go toolchain, so that one makes sense to isolate into a dedicated Alpine script. But the rest of the entire GOOS, GOARCH combinations should be able to be produced with a single gox run.

https://github.com/mcandre/go-ios7crypt