cmd/go: implement the long-promised -run flag for go generate

cmd/go: implement the long-promised -run flag for go generate

Trivial to do, but overlooked for 1.4, which is good because I prefer
the new design, which is just to match against the source code of
the line rather than the command word alone.

Change-Id: Idcf7c4479e97bb7cd732f0d058012321b6057628
Reviewed-on: https://go-review.googlesource.com/9005
Reviewed-by: Russ Cox <rsc@golang.org>