stdlib: Avoid element self-comparisons in qsort

This improves compatibility with applications which assume that qsort
does not invoke the comparison function with equal pointer arguments.

The newly introduced branches should be predictable, as leading to a
call to the comparison function.  If the prediction fails, we avoid
calling the function.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>