Generate platform configuration for all known architectures

Generate platform configuration for all known architectures
- loop over all archs known by rpmrc but only generate the platform config
  if all necessary parameters (such as ISA information) are known, this
  gives a reasonable idea of what archs are *really* in use and supported
- at least in theory, the platform configuration could now move to
  $datadir as the contents no longer depend on which are rpm was built on
- also gets rid of the big sed-monster in install-platform