[RFE] Arch override - allow to exclude platform

[voxik]

Currently the only way to exclude some platform is to list all the supported platforms except the one which should be excluded. But this is suboptimal, since when new arch is added to Koji, the build won't be done on the new platform. Hence, I'd very much prefer, if I can specify e.g. "-ppc64", which would mean build on every architecture except ppc64.