Bug 7458: FIX Snapshot MEP calling convention changes

* sources/dfmc/llvm-back-end/llvm-emit-computation.dylan
  (emit-call): Fix typo
Bug 7458: Add initial code for apply-mep entry point

* sources/dfmc/llvm-back-end/llvm-entry-points.dylan
  (entry-point apply-mep): Implement apply-mep for cases when no argument
   shuffling needs to be done.
Bug 7458: Implement symbol resolution heap fixup

* sources/dfmc/llvm-back-end/llvm-back-end.dylan
  (llvm-heap-fixup-entry-llvm-type): New slot containing the LLVM type
   used for entries in the heap fixup table

* sources/dfmc/llvm-back-end/llvm-back-end-library.dylan
  (module dfmc-llvm-back-end): Export llvm-heap-fixup-entry-llvm-type.

* sources/dfmc/llvm-back-end/llvm-back-end-types.dylan
  (initialize-type-table): Initialize the llvm-heap-fixup-entry-llvm-type
   slot with the necessary struct type.

* sources/dfmc/llvm-back-end/llvm-primitives-symbol.dylan
  (primitive primitive-symbol-fixup): Add a new auxiliary primitive
   for performing symbol fixups based on the heap fixup table.

* sources/app/llvm-runtime-generator/llvm-runtime-generator.dylan
  ($runtime-referenced-functions): Add %resolve-symbol to the set of
   referenced functions.

* sources/dfmc/llvm-linker/llvm-linker.dylan
  ($system-init-fixups-tag): New constant.
  (emit-fixups-elements): New function for computing fixup table entries
   for load-bound references.
  (emit-fixup): New methods for computing fixup addresses for
   <load-bound-reference> entries.
Snapshot threads implementation

* sources/lib/run-time/llvm-posix-threads.c: Fill in more function
  implementations.