The number of CPUs reported by the hw.ncpu sysctl is twice as high as
the actual number of CPUs running on OpenBSD 6.4. with hyperthreading
disabled (hw.smt=0). Try hw.cpuonline first and fall back to hw.ncpu
in case it fails (which is the case on older OpenBSD before 6.4).

Fixes #30127

Change-Id: Id091234b8038cc9f7c40519d039fc1a05437c40d
Reviewed-on: https://go-review.googlesource.com/c/161757
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>