cmd/racebuild: update build instructions
cmd/racebuild: update build instructions

Update llvm's location - it has moved to github.
Turn status messages from "gomote create" off.

Change-Id: I372b43ab4aff23d6657deeb35f3b40a222ca1e73
Reviewed-on: https://go-review.googlesource.com/c/build/+/226541
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build