Introduce %golang_arches_future and stop using %go_arches
    
    This introduces a new %golang_arches_future macro that is the same as
    %golang_arches but without %ix86. This way, packagers can replace
    ExclusvieArch: %go_arches or %golang_arches with it to exclude their
    package from the aforementioned architecture. For packages that use
    %gometa, they simply need to pass the -f flag, which makes the package
    exclusive to %golang_arches_future, instead of %golang_arches.
    
    %gometa was also updated to generate ExclusiveArch: %{golang_arches}
    instead of on %{go_arches}, by default, as %{go_arches} includes
    %{gccgo_arches}, which only includes architectures that Fedora does not
    actually build against.