Fixes the darwin/arm builder (I hope)

Change-Id: I8a3502a1cdd468d4bf9a1c895754ada420b305ce
Reviewed-on: https://go-review.googlesource.com/23684
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>