Shrinks cmd/go by 30KB.

Change-Id: Ied31192e85af76ebac743f8cc12bd9ef6ec5048f
Reviewed-on: https://go-review.googlesource.com/20765Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>