OpenBSD no longer has procfs.

Based on a patch by Matthieu Sarter.

Fixes #19453.

Change-Id: Ia09d16f8a1cbef2f8cc1c5f49e9c61ec7d026a40
Reviewed-on: https://go-review.googlesource.com/46004
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>