unix: use Time{spec,val}.Nano in Time{spec,val}ToNsec
unix: use Time{spec,val}.Nano in Time{spec,val}ToNsec

Follow what CL 188397 did in package syscall:

Call the Nano methods of Timespec and Timeval in TimespecToNsec and
TimevalToNsec respectively, instead of duplicating the implementation.

Change-Id: I50f1958c0f7bca92fc4990684f1de9c148ae5859
Reviewed-on: https://go-review.googlesource.com/c/sys/+/277253
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/sys,golang/sys,golang/sys