On linux/s390x these fields are already hidden. Don't export them on any
other platform either.

Preparatory CL for https://golang.org/cl/106656

Change-Id: I421a0ba41ff46081e2576be212e246a4dd982a67
Reviewed-on: https://go-review.googlesource.com/106838
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>