UPSTREAM PR #17716: ggml-cpu: add RISC-V Vector support for RWKV WKV6 operation

[loci-dev]

Mirrored from ggml-org/llama.cpp#17716

This PR adds RISC-V Vector (RVV) extension support for the RWKV WKV6 operation, enabling vectorized computation on RISC-V platforms.

[loci-review]

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Performance Analysis Summary

Analysis Scope: PR #412 - RISC-V Vector support for RWKV WKV6 operation

Condition Assessment: Condition 1 applies - No meaningful performance impact detected.

Summary

This PR adds RISC-V Vector extension support for RWKV WKV6 operations without affecting inference performance. The only measured change is a 42 ns increase in ggml_compute_forward_solve_tri_f32 (unrelated to PR modifications), which is negligible. No tokenization or inference functions (llama_decode, llama_encode, llama_tokenize) were modified. Power consumption remains stable across all binaries with maximum change of 0.007% in libggml-cpu.so. Tokens per second unaffected.