k_quants : add AVX support to dot functions with QK_K as 64 (#2339)
* add AVX to ggml_vec_dot_q2_K_q8_K()
* add AVX to ggml_vec_dot_q3_K_q8_K()
* add AVX to ggml_vec_dot_q4_K_q8_K()
* add AVX to ggml_vec_dot_q5_K_q8_K()
* add AVX to ggml_vec_dot_q6_K_q8_K()
* refactor AVX code in ggml_vec_dot_q6_K_q8_K()