Bug 7458: Fix bootstrap symbol registration

* sources/dfmc/llvm-back-end/llvm-primitives-symbol.dylan
  (%oblist): Define initial value as the empty vector.
  (op--register-symbol): Store the expanded oblist vector back into %oblist.
Bug 7458: Emit references to engine node entry points

* sources/dfmc/llvm-back-end/llvm-entry-points.dylan
  (llvm-engine-node-ep-info): New auxiliary method for relating
   <&engine-node-ep> objects to the corresponding
   <llvm-entry-point-descriptor>.
  (emit-reference on <&engine-node-ep>): New method to emit references
   to engine node entry point code.
Bug 7458: Emit C struct definitions for runtime-referenced classes

* sources/app/llvm-runtime-generator/llvm-runtime-generator.dylan
  ($runtime-referenced-classes): New constant listing Dylan classes
   referenced within the C runtime
  (generate-runtime-header): Generate struct definitions for each
   of the referenced classes.

* sources/app/llvm-runtime-generator/llvm-runtime-c-header.dylan
  (print-class-c-struct-declaration): New method for generating a C struct
   definition for a <&class>.
Bug 7458: Fix generation of defined-constant references

* sources/dfmc/llvm-back-end/llvm-emit-computation.dylan
  (emit-reference on <defined-constant-reference>): Load the constant
   value from the constant definition global, declaring the value as
   invarant to allow eliminating subsequent loads.
TEMPORARY: call llvm.trap() from unimplemented entry points

* sources/dfmc/llvm-back-end/llvm-entry-points.dylan
  (apply-mep): Return undef
  (rest-key-mep): Call llvm.trap()
  (general-engine-node-3): Return undef
TEMPORARY: Refine skeleton run-time implementation