dylan-compiler: Add a -back-end command line option

* sources/environment/console/command-line.dylan
  (%back-end): New slot in <basic-main-command>
  (do-execute-command on <basic-main-command>): Set the
   compiler-back-end property when the %back-end is set.

* sources/environment/console/compiler-command-line.dylan
  (back-end): New command-line keyword.

* sources/environment/console/environment-command-line.dylan
  (back-end): New command-line keyword.