[opus] fixed point version for celt_pitch_xcorr on aarch64

[Timothy B. Terriberry]

Viswanath Puttagunta wrote:
> Could you please elaborate on "It would be nice to have"? Specifically:
> - Are there use cases where fixed point is preferred when AAarch64 has
> mandatory support for floating point both in regular CPU as well as
> NEON?

Even on x86, when the complexity setting is below the maximum, then for 
medium-low bitrate speech the fixed-point encoder will generally be 
faster, because much of the SILK processing uses exact integer math, and 
this avoids several float->int->float round trips. That's why the x86 
intrinsics code Cisco contributed focused on fixed-point, for example.

I have not tested on aarch64, but I expect similar properties to hold.

It also depends on the rest of your pipeline. If your whole audio 
pipeline is fixed-point (which is common in real-time stacks), then 
you'd have to pay additional conversion penalties to use the 
floating-point API.