2017/state-of-go/stdlib/sort: fix build on 32-bit plaforms

Change-Id: I071e77d972c6fba474e00b2661cc9966bec56826
Reviewed-on: https://go-review.googlesource.com/37297
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>