cmd/go: add *_unix.go recognition (and presumably unix build tag)

It does understand *_windows.go, and I was surprised to (re)learn that it does not support *_unix.go. Would it do so, things would be clearer and all the *_unix.go files that already exist wouldn't need to maintain their

```
// +build darwin dragonfly freebsd linux netbsd openbsd solaris
```

tags as the list grows.