aarch64: Use NEON when SVE width is 128 bits

[AWSjswinney]

On AArch64 systems with SVE support, 128-bit SVE implementations can perform significantly worse than equivalent NEON code due to the different optimization strategies used in each implementation. The NEON version is unrolled 4 times, providing excellent performance at the fixed 128-bit width. The SVE version can achieve similar or better performance through its variable-width operations on systems with 256-bit or 512-bit SVE, but on 128-bit SVE systems, the NEON unrolled implementation is faster due to reduced overhead.

This change adds runtime detection of SVE vector length and falls back to the optimized NEON implementation when SVE is operating at 128-bit width, ensuring optimal performance across all AArch64 configurations.

This implementation checks the vector length with an intrinsic if the compiler supports it (which works on Apple as well) and falls back to using prctl otherwise.

This optimization ensures that systems benefit from:

• 4x unrolled NEON code on 128-bit SVE systems
• Variable-width SVE optimizations on wider SVE implementations
• Maintained compatibility across different AArch64 configurations

Performance improvement on systems with 128-bit SVE:

• Encode: 7509.80 MB/s → 8995.59 MB/s (+19.8% improvement)
• Decode: 9383.67 MB/s → 12272.38 MB/s (+30.8% improvement)