[release-branch.go1.12] syscall: avoid _getdirentries64 on darwin

Getdirentries is implemented with the __getdirentries64 function
in libSystem.dylib. That function works, but it's on Apple's
can't-be-used-in-an-app-store-application list.

Implement Getdirentries using the underlying fdopendir/readdir_r/closedir.
The simulation isn't faithful, and could be slow, but it should handle
common cases.

Don't use Getdirentries in the stdlib, use fdopendir/readdir_r/closedir
instead (via (*os.File).readdirnames).

(Incorporates CL 170837 and CL 170698, which were small fixes to the
original tip CL.)

Fixes #31244

Update #28984

RELNOTE=yes

Change-Id: Ia6b5d003e5bfe43ba54b1e1d9cfa792cc6511717
Reviewed-on: https://go-review.googlesource.com/c/go/+/168479


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 9da6530f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/170640
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>