cmd/goyacc: move go tool yacc from the go repo

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>
5 files changed