Rationale
Free Pascal Compiler (fpc) package is currently only available for primary arches, fails to build on ppc and is excluded from building on arm and s390. All packages that depend on fpc for building must carry a ExcludeArch entry (which violates Guidelines) or manually list supported architectures with Exclusive_Arch entry in .spec file.
In the case in future fpc will be available on more architectures all dependent packages will have to modify their .spec file and do a rebuild.
Proposal
The proposal is to introduce a new dynamic macro which can be used in a Exclusive_Arch entry:
ExclusiveArch: %{fpc_arches}
This way packages can only build on architectures where fpc is available and in future there will be no need of manual intervention if fpc will become available on more architectures.