runtime: introduce and consistently use setNsec for timespec

The general code for setting a timespec value sometimes used set_nsec
and sometimes used a combination of set_sec and set_nsec. Standardize
on a setNsec function that takes a number of nanoseconds and splits
them up to set the tv_sec and tv_nsec fields. Consistently mark
setNsec as go:nosplit, since it has to be that way on some systems
including Darwin and GNU/Linux. Consistently use timediv on 32-bit
systems to help stay within split-stack limits on processors that
don't have a 64-bit division instruction.

Change-Id: I6396bb7ddbef171a96876bdeaf7a1c585a6d725b
Reviewed-on: https://go-review.googlesource.com/c/go/+/167389


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>