This is preliminary work to unifying them.
Aside from Syscall9, all are identical.
Syscall9 has a netbsd/openbsd variant
and a dragonfly/freebsd variant.

Updates #11041

Change-Id: Ia5ce95d5e9115d4c0492d5e53aa7a4316deafd1f
Reviewed-on: https://go-review.googlesource.com/25115
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>