This moves the 'go tool yacc' command from the main Go repo
to x/tools.
Copied from go rev 795ad07b3 + doc changes from "go tool yacc" to "goyacc".
Updates golang/go#11229
Change-Id: I6d17911a3bf64724c090c4fe4903238e3bce3b8c
Reviewed-on: https://go-review.googlesource.com/27324
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
