I'm not aware of any official or standard definition of system programming. There's application programming and anything which doesn't fit in there is system programming which can be low-level, middleware, etc.

Go is appropriate for writing services / daemons, command line tools, servers, etc. It's not well suited for writing operating system code and very performance-intensive code (databases, audio/video, etc), but there's still a big overlap with Rust.

It seems to me that this whole discussion keeps coming up because parts of the Rust community are worried about the competition. And they have reason to be :-)