os: don't use wait6 on netbsd

CL 315281 changed the os package use wait6 on netbsd. This seems to be
causing frequent test failures as reported in #48789. Revert that change
using wait6 on netbsd for now.

Updates #13987
Updates #16028
For #48789

Change-Id: Ieddffc65611c7f449971eaa8ed6f4299a5f742c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/354249


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>