macOS Sierra beta4 changed the kernel interface for getting time.
DX now optionally points to an address for additional info.
Set it to zero to avoid corrupting memory.

Fixes #16570

Change-Id: I9f537e552682045325cdbb68b7d0b4ddafade14a
Reviewed-on: https://go-review.googlesource.com/25400Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>