unix: add Auxv

Fixes golang/go#67839

Change-Id: I3af38d21159f7cac3786b49ac17657d314fbc178
Reviewed-on: https://go-review.googlesource.com/c/sys/+/644295
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>