Add function TimeToTimespec for converting time.Time to Timespec.

Fixes golang/go#23078.

Change-Id: I685a4871d49131191f330b56b15bfd4f3131a9a5
Reviewed-on: https://go-review.googlesource.com/82919
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>