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.