Revert "Use std::stable_sort() instead of std::sort()"

Unfortunatly std::stable_sort() implementation in gcc is
horrendously slow. We have a big performance regression on
Linux systems (-20% !)

So revert the commit and wait to fix the issue in a different
way, perhaps with an our home grown sorting, that should be
comparable in speed with std::sort()

Signed-off-by: Marco Costalba <mcostalba@gmail.com>