[opus] [RFC V3 7/8] armv7, armv8: Optimize fixed point fft using NE10 library

[Timothy B. Terriberry]

I'm trying to get these cleaned up and landed, but I'm running into  
some trouble with this patch. Using commit a08b29d88e3c (July 21) of  
Ne10, I'm seeing test failures for 60-point FFTs:

nfft=60 inverse=0,snr = -3.312408
** poor snr: -3.312408 **
nfft=60 inverse=1,snr = -16.079597
** poor snr: -16.079597 **

All other sizes tested appear to work fine (84 to 140 dB of SNR). This  
doesn't match the results you reported on this list in May, where the  
FFTs appeared to pass tests, but the MDCTs did not (the latter of  
which I presume was fixed by the May 12 commits to Ne10). I don't see  
any commits since then that I would expect to have broken this. Any  
ideas?