llamafile: add rvv support for sgemm kernels

[taimur-10x]

Summary

This PR adds RISC-V vector (RVV) support for the SGEMM kernels.

These kernels are used by GGML_OP_MUL_MAT for prompt processing.

Key Changes

• RVV Support is added for F16 and F32 types (with the zvfh extension) and BF16 (with the zvfbfwma extension).
• Tiling was decided based on various LMUL configurations
  • 4x6 with LMUL=1 (32 register groups)
  • 4x3 with LMUL=2 (16 register groups)
  • 2x2 with LMUL=4 (8 register groups)

Testing

Kernels were functionally tested on QEMU for VLENs (128-bit, 256-bit, 512-bit and 1024-bit) for a range of input sizes.

Benchmarking Results

End-to-end benchmarking on BananaPI-BPI F3 (VLEN=256)

Prefill / Prompt Processing

Tokens / Second

Model	Prompt Size	SGEMM 4x6	SGEMM 4x3	SGEMM 2x2	Vector Dot
Tinyllama F16 1.1B	32	6.08	7.89	6.26	8.42
Tinyllama F16 1.1B	64	6.09	7.25	11.31	7.57
Tinyllama F16 1.1B	128	5.93	6.9	13.73	8.78
Tinyllama F16 1.1B	256	5.54	6.79	12.56	8.57
Tinyllama F16 1.1B	512	5.37	6.64	13.37	8.68

[luhenry]

@xctan it would be lovely to have your review on that as it will go to upstream next.

[xctan]

LGTM.