Reduces binary size of cmd/go by 0.5%.
For #6853.

Change-Id: I5a4b814049580ab5098ad252d979f80b70d8a5f9
Reviewed-on: https://go-review.googlesource.com/19694Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>