[libc] Add qsort_r

This patch adds the reentrent qsort entrypoint, qsort_r. This is done by
extending the qsort functionality and moving it to a shared utility
header. For this reason the qsort_r tests focus mostly on the places
where it differs from qsort, since they share the same sorting code.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D152467