Reduce call chain of qsort

This will considerably improve the performance of qsort-heavily-used projects
like Android-Pinyin-IME, more discussion:
  https://groups.google.com/forum/#!topic/emscripten-discuss/Ah2CNfQ8ra4
  https://github.com/kripken/emscripten/pull/1437
Merge branch 'qsort_reduce_callchain' of github.com:PinZhang/emscripten into incoming