Bug 7458: Add varargs support to dfmc-llvm-back-end

* sources/dfmc/llvm-back-end/llvm-back-end.lid: Add llvm-varargs.dylan to the
  library build.

* sources/dfmc/llvm-back-end/llvm-varargs.dylan: New source file containing
  varargs support routines.

* sources/dfmc/llvm-back-end/llvm-ops.dylan
  (op--stack-allocate-vector): New utility method for stack-allocating
   an instance of <simple-object-vector>.
Bug 7458: Generate entry-point functions for LLVM

* sources/dfmc/llvm-back-end/llvm-emit-code.dylan
  (llvm-calling-convention on <&shared-entry-point>): Use the C
   calling convention for XEP and MEP functions.

* sources/dfmc/llvm-back-end/llvm-entry-points.dylan
  ($key-mep-string): Rename to $rest-key-mep-string, and change name
  to match the one used by HARP.
  (<llvm-entry-point-descriptor>): New class for representing
   entry-point definitions.
  (entry-point-descriptor-definer, entry-point-emitter-method): New
   macros for defining entry points.
  ($llvm-entry-point-descriptors): New unexported constant.
  (do-define-llvm-entry-point-descriptor): New unexported auxiliary function.
  (llvm-entry-point-function): New method for returning a <llvm-function>
   for an entry-point.
  (make-entry-point-function) New unexported auxiliary function.
  (llvm-entry-point-rest?): New method for returning whether or not
   an entry point is declared as taking a #rest argument.
  (entry-point-rest-test): New unexported auxiliary function.
  (llvm-entry-point-function-type): New method for returning the
   <llvm-function-type> for an entry-point.
  (op--call-error-iep): New auxiliary method for generating a call to
   a named function in the Dylan library to signal a nonrecoverable
   error.
  (op--argument-count-error): New method to generate the signalling of
   <argument-count-error>.
  (op--odd-keyword-arguments-error): New method to generate the
   signalling of <odd-keyword-arguments-error>.
  (op--type-check-lambda-arguments): New method to generate type
   checks of a function's arguments against its declared signature.
  (entry-point apply-xep): New auxiliary entry point for implementing apply().
  (entry-point xep, rest-xep, gf-xep, gf-optional-xep): New entry
   point definitions.
  (entry-point rest-key-xep, rest-key-mep): New stubbed-out entry
   point definitions, not yet implemented.

* sources/dfmc/llvm-back-end/llvm-back-end-library.dylan
  (module dfmc-llvm-back-end): Export new entry-point related
   definitions.

* sources/app/llvm-runtime-generator/llvm-runtime-generator.dylan
  ($runtime-referenced-functions): New unexported constant listing function
   names in the Dylan library that need to be imported into the runtime.
  (generate-runtime-heap): Add code to import Dylan library functions.
  (generate-runtime-entry-point): New auxiliary function for generating
   entry-point functions and adding them to the runtime.
  (generate-runtime): Invoke generate-runtime-entry-points.