* ggml-cpu: Faster IQ1 mul_mat_vec on AVX2 using BMI2 instructions
* cmake: Add GGML_BMI2 build option
* ggml: enable BMI2 on relevant CPU variants
* ggml-cpu: include BMI2 in backend score
* ggml-cpu: register BMI2 in ggml_backend_cpu_get_features
* ggml-cpu: add __BMI2__ define when using MSVC