OpenBSD uses sysctl with struct uvmexp to get information from the
virtual memory system of the kernel. Add type Uvmexp and the
SysctlUvmexp function to query this information.

This will be used in github.com/tklauser/go-sysconf to get
_SC_AVPHYS_PAGES on OpenBSD.

Change-Id: I96ded2d1be37e5307bab55e79b13234cc93d21e6
Reviewed-on: https://go-review.googlesource.com/c/139278
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>