[opus] NEON detection under iOs

[Victor Cherepanov]

Hi,

Thank you for a great work. Opus codec works great under many platforms.
I have found a small performance issue under iOS platform.
If the macro OPUS_HAVE_RTCD is not set, then encoder doesn't use some _neon
functions at low bitrates (up to 64k). If the macro is set, then the
compiler hits the error at opus/celt/arm/armcpu.c:153 (a function for
detection missed). Being compared to Android version performance
degradation is ~30%.
Detection NEON under iOS is a bit tricky, because there is no API for it. I
added compiler-time detection.
I made a commit into github repository. Attached please find patch for it.

Regards,
Victor
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