[Go] use GOOS to distinguish between platforms

[wamuir]

Running go generate within tensorflow/go/op executes tensorflow/go/genop/generate.sh superfluously (twice). This is because tensorflow/go/genop/generate.win.go does not follow golang build contraints which, for file suffixes, require an underscore and the full OS name of windows (e.g., generate_windows.go). Thus, go generate runs tensorflow/go/genop/generate.go and tensorflow/go/genop/generate.win.go on any platform.

It is more direct to remove the build constraint and instead distinguish between platforms within the shell script as go env GOOS provides the platform name. This also allows the script to be called directly with GOOS set as an environment variable.