dfmc: Disable compiling HARP on platforms it does not support

* sources/dfmc/back-end-implementations/library.dylan
  (library dfmc-back-end-implementations): Remove references to
   dfmc-harp-x86-cg and dfmc-harp-browser-support.

* sources/dfmc/back-end-implementations/library-harp.dylan: Copied from
  library.dylan prior to the removal references to HARP back-end libraries.

* sources/dfmc/back-end-implementations/back-end-implementations-harp.lid:
  New LID file referencing library-harp.dylan

* sources/registry/x86-freebsd/dfmc-back-end-implementations,
  sources/registry/x86-linux/dfmc-back-end-implementations,
  sources/registry/x86-win32/dfmc-back-end-implementations: Reference
  library-harp.dylan on platforms that HARP does support.