Reduces best of 10 linking of cmd/go by ~5%

Change-Id: If673b877ee12595dae517d7eb48430451e5cadba
Reviewed-on: https://go-review.googlesource.com/20060Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>