cmake HAVE_M_AVX2_FLAG examples

pingcap/tiflash contrib/aws-cmake/AwsSIMD.cmake :9 

check_c_compiler_flag("/arch:AVX2" HAVE_M_AVX2_FLAG)

pingcap/tiflash contrib/aws-cmake/AwsSIMD.cmake :14 

check_c_compiler_flag(-mavx2 HAVE_M_AVX2_FLAG)