cmake AWS_HAVE_MM256_EXTRACT_EPI64 examples

awslabs/aws-c-common cmake/AwsSIMD.cmake :72 

check_c_source_compiles("
        #include <immintrin.h>
        #include <string.h>

        int main() {
            __m256i vec;
            memset(&vec, 0, sizeof(vec));
            return (int)_mm256_extract_epi64(vec, 2);
        }" AWS_HAVE_MM256_EXTRACT_EPI64)