QNX: Make use of _readdir64_r

Latest QNX versions support _readdir64_r. It will be used by
QFileSystemIterator now, when available to support large file
systems.

Backport from qtbase/5d8c05baf478d8eb8cb7ce827caa2c1103f5fa3f

Change-Id: I264cc39a0cae6e84ed0db2e10df5ea22c8ae3df4
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>