Companion PR to https://github.com/golang/sys/pull/13

Change-Id: I097fc97912840eb69ca232eded6ba939de0fead9
GitHub-Last-Rev: f8a8f7d96c96e3cb03010cb3d9607741f4bbc3a1
GitHub-Pull-Request: golang/go#26675
Reviewed-on: https://go-review.googlesource.com/126621
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>