gcc: Allow -mtune to also be set (UNSAFE).

This option reuses the -march menu entirely and allows
you to change the -mtune value as well, which is handy for
people building ISO's and binaries for other systems.

reminder:

-march => will not work for anything else but $value and newer
-mtune => will be optimized for $value

ISO creators will want to do something like:

ix86:
-march=i586 -mtune=core2

x86_64:
-march=<unset> -mtune=<unset>

ISO creators should NEVER set -march=native. Ever.