The getdirentries syscall is considered private API on iOS and is
rejected by the App Store submission checks. Replace it with the
fdopendir/readdir_r/closedir syscalls.

Fixes #28984

Change-Id: I73341b124310e9cb34834a95f946769f337ec5b7
Reviewed-on: https://go-review.googlesource.com/c/153338Reviewed-by: Keith Randall <khr@golang.org>