Use more MAC16_16's and unroll a loop.

This splits out the non-arch-specific portions of a patch written
 by Aurélien Zanelli <aurelien.zanelli@parrot.com
 http://lists.xiph.org/pipermail/opus/2013-May/002088.html

I also added support for odd n, for custom modes.

0.25% speedup on 96 kbps stereo encode+decode on a Cortex A8.