Do not treat "Class" tag as a fully arbitrary tag.

     Change: make-class-available-as-macro
     Purpose: Do not treat "Class" tag as a fully arbitrary tag.
     Reason: In OpenPKG some .spec processing depends on the
                     macro %{class} to be filled from the value of the
                     "Class" tag, hence make sure it is not treated fully
                     arbitrary (or else the macro would be not available).
     Hint: As %_arbitrary_tags restricts the allowed tags anyway,
                     best would be to export also arbitrary tags as macros!