Also consolidate platform-dependent helper files.

Updates golang/go#17015.

Change-Id: Ibf09479762029ecc7229ab4bb233138e0d4e69d9
Reviewed-on: https://go-review.googlesource.com/28997
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>