* frame.h (frame_unwind_arch): New.
(frame_unwind_caller_arch): Likewise.
* frame-unwind.h (frame_prev_arch_ftype): New type.
(struct frame_unwind): New member prev_arch.
* frame.c (struct frame_info): New member prev_arch.
(frame_unwind_arch): New function.
(frame_unwind_caller_arch): Likewise..
(get_frame_arch): Reimplement in terms of frame_unwind_arch.
* sentinel-frame.c (sentinel_frame_prev_arch): New function.
(sentinel_frame_unwinder): Install it.