Fix uninitialized register read problem.

Fix uninitialized register read problem.

* builtins.c (expand_builtin_return_addr): Set
current_function_accesses_prior_frames when count != 0.  Use
frame_pointer_rtx when count == 0.
* function.h (struct function): Add accesses_prior_frames field.
(current_function_accesses_prior_frames): Define.
* reload1.c (init_elim_table): Check
current_function_accesses_prior_frames.
* doc/tm.texi (INITIAL_FRAME_ADDRESS_RTX): Update docs.

From-SVN: r103294