os: use syscall.Timespec.Unix

os: use syscall.Timespec.Unix

Use the syscall method instead of repeating the type conversions for each OS.

Change-Id: I1db975b3aaa189cf724d7b1b7c5c41bc64dd964d
Reviewed-on: https://go-review.googlesource.com/c/go/+/374574
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>