[opus] fixed point version for celt_pitch_xcorr on aarch64

[Timothy B. Terriberry]

Zhongwei Yao wrote:
> Hi, all,
>
> Does Opus need celt_pitch_xcorr’ s fixed point version for ARM aarch64
> architecture? If yes, which version does Opus prefer: assembly or
> instrinsics?

It would be nice to have one. I don't have a lot of experience with 
aarch64 (I still haven't been able to obtain a dev board), so I don't 
really know how intrinsics compare to assembly. Historically, intrinsics 
performance on most platforms has been significantly below that of 
hand-written assembly, and the tool support is more of a headache, which 
is why we've favored hand-written assembly, but getting some kind of 
vectorization is better than the serial code we currently have.