resample: Add NEON optimized inner_product_single for fixed point

Semantics of inner_product_single have also been changed to contain
the final right shift and saturation so it can also be implemented in
the optimal way for the used platform. This change affects fixed point
calculations only.

I also added a new fixed point macro SATURATE32PSHR(x, shift, a). It
does pretty much the same thing as SATURATE32(PSHR32(x, shift), a),
but it avoids over flowing in rounding up phase in the rare occasion
where x has already been saturated. It should also be slightly faster.