stdlib: Optimization qsort{_r} swap implementation

The optimization takes in consideration both the most common elements
are either 32 or 64 bit in size and inputs are aligned to the word
boundary.  This is similar to what msort does.

For large buffer the swap operation uses memcpy/mempcpy with a
small fixed size buffer (so compiler might inline the operations).

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