runtime: fix nanotime for macOS Sierra

This is a cherry-pick of https://go-review.googlesource.com/24812
to the release-branch-go1.4

In the beta version of the macOS Sierra (10.12) release, the
gettimeofday system call changed on x86. Previously it always returned
the time in the AX/DX registers. Now, if AX is returned as 0, it means
that the system call has stored the values into the memory pointed to by
the first argument, just as the libc gettimeofday function does. The
libc function handles both cases, and we need to do so as well.

Fixes #16272.

Change-Id: I490ed0a82e251fce73becc4722cbe276feebc7b7
Reviewed-on: https://go-review.googlesource.com/31729


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