ggml-cpu:fix RISC-V Q4_0 repack select and RVV feature reporting

[ixgbe]

Changes included:

• Add ggml_cpu_get_rvv_cnt() and RVV vector-length initialization.
• Export RVV_CNT in CPU feature list.
• Update ggml_repack_get_optimal_repack_type() to enable Q4_0 repack when
  ggml_cpu_has_riscv_v() and rvv_cnt >= QK4_0.

[xctan]

I suggest using the name VLEN instead of copying the name CNT from SVE svcntX instruction families.