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ARROW-11839: [C++] Use xsimd for generation of accelerated bit-unpacking

The custom per-ISA code generation scripts (AVX2, AVX512) are replaced with a single code generation script that outputs xsimd code for any SIMD bit-width, in an ISA-agnostic way.

Also add a Neon optimized version of bit-unpacking that leverages the generated code for 128-bit SIMD.

Closes #9614 from pitrou/ARROW-11839-xsimd-bpacking

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>