* guix/packages.scm (%supported-systems): New variable.
  (<package>)[platforms]: Rename to...
  [supported-systems]: ... this.  Change default to %SUPPORTED-SYSTEMS.
* build-aux/hydra/gnu-system.scm (job-name, package->job): New
  procedures, formerly in 'hydra-jobs'.  Honor 'package-supported-systems'.
  (hydra-jobs): Use them.