better RTL debugging

better RTL debugging

* libguile/frames.c (scm_frame_source, scm_frame_instruction_pointer):
  Fix to work with RTL programs.

* module/system/vm/debug.scm (find-debug-context): Allow for the
  possibility of there being no ELF image.
  (find-program-debug-info, find-program-arities)
  (program-minimum-arity, find-program-docstring)
  (find-program-properties, find-source-for-addr)
  (find-program-die, find-program-sources): Don't bail if we couldn't
  get the debug context.

* module/system/vm/frame.scm (frame-next-source)
  (frame-call-representation): Allow RTL programs.

* module/system/vm/program.scm (program-arguments-alist): Placeholder
  implementation for RTL programs.
  (program-arguments-alists): Don't bail if we couldn't get the
  arities.