runtime: fix nanotime for macOS Sierra, again.

This is a cherry-pick of https://go-review.googlesource.com/25400
to the release-branch-go1.4

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: I714325a7749a145d23cf03251db38196ac9c481a
Reviewed-on: https://go-review.googlesource.com/31750


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>