ggml-cpu: Faster IQ1 mul_mat_vec on AVX2 using BMI2 instructions (#12154)

* 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

ggml/src/ggml-cpu/cpu-feats-x86.cpp

@@ -278,6 +278,10 @@ static int ggml_backend_cpu_x86_score() {
     if (!is.SSE42()) { return 0; }
     score += 1<<2;
 #endif
+#ifdef GGML_BMI2
+    if (!is.BMI2()) { return 0; }
+    score += 1<<3;
+#endif
 #ifdef GGML_AVX
     if (!is.AVX()) { return 0; }
     score += 1<<4;