stdlib: Implement introsort for qsort (BZ 19305)

This patch makes the quicksort implementation to acts as introsort, to
avoid worse-case performance (and thus making it O(nlog n)).  It switch
to heapsort when the depth level reaches 2*log2(total elements).  The
heapsort is a textbook implementation.

Checked on x86_64-linux-gnu and aarch64-linux-gnu.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>