runtime: consolidate on a single closeonexec definition

runtime: consolidate on a single closeonexec definition

Now that we implement fcntl on all Unix systems, we can
write closeonexec that uses it. This lets us remove a bunch
of assembler code.

Change-Id: If35591df535ccfc67292086a9492f0a8920e3681
Reviewed-on: https://go-review.googlesource.com/c/go/+/496081
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>