libavutil/softfloat: Added av_normalize_sf in av_add_sf

This will normalize sums for which mantissa is smaller than the lower boundary
(needed for implementation of fixed point aac decoder).

Signed-off-by: Nedeljko Babic <nedeljko.babic@imgtec.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>