Bug 7458: Add exception handling struct to bind-exit frames

* sources/dfmc/llvm-back-end/llvm-targets.dylan
  (llvm-back-end-unwind-exception-size): New generic returning the rounded-up
   number of words in an unwinder struct _Unwind_Exception.

* sources/dfmc/llvm-back-end/llvm-primitives-nlx.dylan
  (initialize-bef-struct-type): Add a field to hold struct _Unwind_Exception
   within a BEF (bind-exit frame).
Bug 7458: Implement run-time for LLVM non-local exits

* sources/lib/run-time/llvm-nlx.c
  (CONTAINER_OF, OPENDYLAN_EXCEPTION_CLASS, DW_EH_PE_*): New definitions.
  (get_ULEB128, get_SLEB128, get_encoded_size, get_encoded,
   opendylan_exception_cleanup): New static functions.
  (primitive_nlx): Implement unwinding using _Unwind_RaiseException().
  (handler_found): New static function.
  (__opendylan_personality_v0): Implement dispatch to dfmc-llvm-back-end
   generated exception landing pads for <bind-exit> and <unwind-protect>.
Bug 7458: Fix function call generation

* sources/dfmc/llvm-back-end/llvm-emit-computation.dylan
  (emit-call): Reference unknown function values via c.function
   rather than c.call-effective-function.