Rollup merge of #32342 - petevine:patch-1, r=alexcrichton

Use explicit -march flags in the i586 mk file

`-march` should definitely go last, after the environment `$C(XX)FLAGS`, or it's going to remain brittle.

This should fix cross-compilation issues on x86_64 (possibly x86 too on some newer distros) - it was far too assuming of me to expect you really had to want i586 using your own flags.