frame, backtrace: allow targets to supply a frame unwinder

frame, backtrace: allow targets to supply a frame unwinder

Allow targets to supply their own target-specific frame unwinders; one for
normal frames and one for tailcall frames.  If a target-specific unwinder
is supplied, it will be chosen before any other unwinder.

The original patch has been split into this and the next two patches.

gdb/
2013-02-11  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* frame-unwind.c: Include target.h.
	(frame_unwind_try_unwinder): New function with code from ...
	(frame_unwind_find_by_frame): ... here.  New variable
	unwinder_from_target, call also target_get_unwinder)
	(target_get_tailcall_unwinder, and frame_unwind_try_unwinder for it.
	* target.c (target_get_unwinder, target_get_tailcall_unwinder): New.
	* target.h (struct target_ops): New fields to_get_unwinder and
	to_get_tailcall_unwinder.
	(target_get_unwinder, target_get_tailcall_unwinder): New declarations.