Just call syscall's implementation instead of duplicating ParseDirent
and the GOOS-specific private functions needed by it.

Change-Id: Icf49769390929463323608e093dd2a3d1c0ae4d3
Reviewed-on: https://go-review.googlesource.com/88475
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>