About llvm, there's a WIP prototype that doesn't quite work yet[0].
And about GCC, GccGo is behind in Go's version. GCC 7 only supports Go 1.8 (with caveats). Also, it had issues at some point[2] (but this is from 2014, it may have been fixed since then).
And, on a more pragmatic point of view, when talking about Go, the huge majority talks about Go compiled with the standard Go compiler. That's why you here a lot of positive feedback about compile-times for instance.
