Speeds up linking cmd/go by 1.7%

name       old s/op   new s/op   delta
LinkCmdGo  0.58 ± 4%  0.57 ± 5%  -1.74%  (p=0.000 n=96+97)

Change-Id: I7844cf4e2eeac260318de2b6ddf52ce07a6e00f5
Reviewed-on: https://go-review.googlesource.com/20915
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>