neon: add dotprod dispatch implementation

* read vector at runtime
     * add enum

cmake/checks/cpu_dotprod.cpp

@@ -0,0 +1,24 @@
+#include <stdio.h>
+
+#if defined __GNUC__ && (defined __arm__ || defined __aarch64__)
+#include "arm_neon.h"
+int test()
+{
+    const unsigned int src[] = { 0, 0, 0, 0 };
+    unsigned int dst[4];
+    uint32x4_t v_src = *(uint32x4_t*)src;
+    uint8x16_t v_m0 = *(uint8x16_t*)src;
+    uint8x16_t v_m1 = *(uint8x16_t*)src;
+    uint32x4_t v_dst = vdotq_u32(v_src, v_m0, v_m1);
+    *(uint32x4_t*)dst = v_dst;
+    return (int)dst[0];
+}
+#else
+#error "DOTPROD is not supported"
+#endif
+
+int main()
+{
+  printf("%d\n", test());
+  return 0;
+}