Bug 7458: Reorganize llvm-back-end entry-point code

* sources/dfmc/llvm-back-end/llvm-emit-object.dylan: Move function
  entry-point reference emitter definitions from here.

* sources/dfmc/llvm-back-end/llvm-entry-points.dylan: New file,
  containing moved definitions.

* sources/dfmc/llvm-back-end/llvm-back-end.lid: Reference new file.