Patch up runtime testing to use the libc fcntl function on Darwin,
which is what we should be doing anyhow. This is similar to how
we handle fcntl on AIX and Solaris.

Fixes #36211

Change-Id: I47ad87e11df043ce21496a0d59523dad28960f76
Reviewed-on: https://go-review.googlesource.com/c/go/+/212299
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>