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.
Bug 7458: Build the LLVM run-time on platforms that support it

* configure.ac: Check whether or not the platform supports building
  the LLVM run-time.

* Makefile.in: Build llvm-runtime-generator when it is needed for
  bootstrapping. Pass the runtime-generator path as
  OPEN_DYLAN_LLVM_RUNTIME_GENERATOR when invoking the run-time build.

* sources/lib/run-time/Makefile.in: When a OPEN_DYLAN_LLVM_RUNTIME_GENERATOR
  path is supplied, build and install the run-time for the LLVM back-end.