runtime: fix sysctl calling convention on netbsd/386

Thanks to coypoop for noticing at:

  https://github.com/golang/go/issues/22914#issuecomment-347761838

FreeBSD/386 and NetBSD/386 diverged between Go 1.4 and Go 1.5 when
Russ sent https://golang.org/cl/135830043 (git rev 25f6b02a)
to change the calling convention of the C compilers to match Go.
But netbsd wasn't updated.

Tested on a NetBSD/386 VM, since the builders aren't back up yet (due
to this bug)

Fixes #22914
Updates #19339
Updates #20852
Updates #16511

Change-Id: Id76ebe8f29bcc85e39b1c11090639d906cd6cf04
Reviewed-on: https://go-review.googlesource.com/80515


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>