go: Correctly build (cross-)compilers

Without passing the --no-clean flag to make.bash, each build will
overwrite the previous one. This means that we always ended up with a
single compiler, that for x86_64.
go: include godoc, vet and cover

godoc, vet and cover are part of the official Go distribution, but live
in repositories separate from Go itself.
go: change maintainer
go: build for all supported OSs
Merge pull request #453 from dominikh/fix_go

Improve Go package