Support a fixed-point build

[jwcullen]

It's common for this type of software to be configurable to work better on fixed-point systems.

• https://github.com/xiph/opus/blob/main/README#L166-L171

There may be other parts that are floating point, but at least demixing module and resampler only use floating point.

I think this could be a big task to fix, but it may make the software more useful for use on certain systems.

[yilun-zhangs]

@jwcullen
Yes, it may require fixed-point version reference for certain systems,
and this is a big task to support fixed-point option, we will enhance this next, it may take long time to proceed.