Add the MNT_* constants used for the Mount and Umount functions on
Darwin.

Generated on Mac OS 10.11 (Darwin 15.6.0). This also brings
zerrors_darwin_{arm,arm64} up to date with constants which are already
present for 386 and amd64.

Fixes golang/go#9891

Change-Id: Id77847a2fb805a759aa0f5b36e8954b764e699a5
Reviewed-on: https://go-review.googlesource.com/54210Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>