ggml : IQ3_S improvements (#5829)

ggml : IQ3_S improvements (#5829)

* iq3_s: somewhat faster AVX2 dot product

On Ryzen a 7950X TG-128 increases to 16 t/s from 15.5 t/s using
16 threads. For 8 threads it is 13.85 t/s vs 11.75 t/s.
PP-512 increases to 28.5 t/s from 23.8 t/s.

* iq3_s: somewhat faster ARM_NEON dot product

Still dog slow - 10.7 t/s up from 9.9 t/s.

* iq3_s: another small ARM_NEON improvement

10.7 -> 11.0 t/s. Using vmulq_s8 is faster than the xor - sub trick
that works best on AVX2.

* iq3_s: minor improvement on Metal

49.4 t/s -> 50.3 t/s

* iq3_s: PPL improvement

E.g., for a context of 4096 LLaMA-v2-7B goes to 5.1340 from 5.1653.

* iq3_s: use new grid everywhere

* Fix ARM_NEON

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Co-authored-by: Iwan Kawrakow <redacted>