syscall: minor cleanup of duplicated code

syscall: minor cleanup of duplicated code

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

Change-Id: I17551ea54c59c1e45ce472e029c625093a67251a
GitHub-Last-Rev: fecf43d163f4ebe72e8bb1d3854d4ad962c08b03
GitHub-Pull-Request: golang/go#33390
Reviewed-on: https://go-review.googlesource.com/c/go/+/188397
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>