unix: define IfMsghdr2, IfData64, and RtMsghdr2 on darwin

Change-Id: I6943f6b5bd9f7d0ef5bad24ed638b8cf5dd0353d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/633077
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>