GNU Compatibility: Implement qsort_r

This is a version of the qsort function allowing you to pass a "cookie" to the function.

Change-Id: I60c645213b9c9590e38b112634fcac1d7969b6d9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2449
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>