It's not guaranteed that the []byte buffer will be aligned as
required for Clockinfo. Use a Clockinfo var for the sysctl call
instead.

This came up during the review for SysctlUvmexp on OpenBSD in CL
139278. Thanks to Ian Lance Taylor for pointing this out.

Change-Id: Idc7a624922da7249c6e7d5ce0236a431b58ebe5f
Reviewed-on: https://go-review.googlesource.com/c/139279
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>