silk: Add Arm NEON silk_VAD_GetSA_Q8

[czoli1976]

Summary

silk_VAD_GetSA_Q8 had an x86 SSE4.1 implementation but no Arm one, even though it runs on every SILK/hybrid frame in the default (float) build. This adds a NEON version, mirroring the SSE4.1 one.

What's vectorised

The per-subframe energy sum-of-squares — (X[i] >> 3)^2 accumulated in int32 — 8 samples per iteration via vshrq_n_s16 + paired vmlal_s16 (low/high), with a scalar tail and a horizontal vaddvq_s32. Everything else (analysis filterbank, noise estimation, SNR/tilt) is identical to the C reference, exactly as the SSE4.1 version does.

• Bit-exact with silk_VAD_GetSA_Q8_c (exact integer sum of squares, no overflow), validated by the existing OPUS_CHECK_ASM full-encoder-state memcmp.
• As on x86, silk_VAD_GetNoiseLevels becomes exported (instead of static inline in VAD.c) when NEON is enabled, so the kernel can call it.

Dispatch / wiring

Uses the existing OVERRIDE_silk_VAD_GetSA_Q8 hook: a new silk/arm/VAD_arm.h provides the PRESUME (direct call) and RTCD (SILK_VAD_GETSA_Q8_IMPL table in arm_silk_map.c) dispatch, mirroring silk/x86/main_sse.h. The source is added to the common SILK_SOURCES_ARM_NEON_INTR group, which is already wired in autotools / CMake / Meson — so no build-system changes are needed.

Numbers (Apple M4)

• Microbench of the vectorised loop at the real subband lengths (10–80 samples): ~1.1–1.7× over scalar.
• End-to-end within run-to-run noise (VAD is a small per-frame cost), bitstream unchanged (bit-exact).
• Full meson test suite passes.

This is the last of the silk-side x86-has-it/ARM-doesn't parity gaps that runs in the default build.