This pulls in CL 179317 which fixes the build on aix/ppc64.

Change-Id: I7d90950800627904503db97cfb0bc670a45b701e
Reviewed-on: https://go-review.googlesource.com/c/go/+/179357
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>