The existing darwin/amd64 implementation of runtime.nanotime returns the
wallclock time, which results in timers not functioning properly when
system time runs backwards. By implementing the algorithm used by the
darwin syscall mach_absolute_time, timers will function as expected.

The algorithm is described at
https://opensource.apple.com/source/xnu/xnu-3248.60.10/libsyscall/wrappers/mach_absolute_time.s

Fixes #17610

Change-Id: I9c8d35240d48249a6837dca1111b1406e2686f67
Reviewed-on: https://go-review.googlesource.com/35292Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>