cmake AWS_HAVE_AVX2_INTRINSICS examples

awslabs/aws-c-common cmake/AwsSIMD.cmake :48

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

        int main() {
            __m256i vec;
            memset(&vec, 0, sizeof(vec));

            _mm256_shuffle_epi8(vec, vec);
            _mm256_set_epi32(1,2,3,4,5,6,7,8);
            _mm256_permutevar8x32_epi32(vec, vec);

            return 0;
        }"  AWS_HAVE_AVX2_INTRINSICS)