x/sys/unix: make ReadDirent available on zOS

Currently ReadDirent is not available on zOS. This implementation modifies the current darwin implementation to work on zOS.

Also make tests available on zOS.

Fixes golang/go#54528
Fixes golang/go#54587

Change-Id: I62a09d0068ec8c5fdc849a411ce43bdf14ca3926
Reviewed-on: https://go-review.googlesource.com/c/sys/+/424778
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>