Introduce optimized NEON implementations for audio peak detection,
buffer mixing, scaling, and copy functions on AArch64.
For benchmark results, see ardour-neon-functions [1].
Provide a dedicated `aarch64_neon_functions.cc` for Aarch64 target.
provide backwards compatibility with armhf targets with
`arm_neon_fuctions.cc`.
[1] https://github.com/ashafq/ardour-neon-functions