cc: -Xarch should not skip the next argument.

The format is `-Xarch_i386` as opposed to `-Xarch i386` unliked `-arch i386` so the next argument should not be skipped. This was causing issues for some formulae e.g. `qt` in which case it was skipping `-m3dnow` and `-mmmx`.

Also, remove Qt's superenv hacks now that this is fixed.