runtime: use clock_gettime instead of gettimeofday on darwin

clock_gettime has higher resolution than gettimeofday and is available
since macOS 10.12. Go 1.15 already requires at least macOS 10.12 and
thus clock_gettime can be used unconditionally (also see
https://golang.org/doc/go1.15#darwin)

Fixes #25633

Change-Id: I46305387212735e5d3a13e5f02ec90f3e6d546a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/270918


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>