convert to_get_unwinder and to_get_tailcall_unwinder to methods

convert to_get_unwinder and to_get_tailcall_unwinder to methods

This converts to_get_unwinder and to_get_tailcall_unwinder to methods
and arranges for them to use the new delegation scheme.

This just lets us avoid having a differing style (neither new-style
nor INHERIT) of delegation in the tree.

2014-02-19  Tom Tromey  <tromey@redhat.com>

	* target.c (target_get_unwinder): Rewrite.
	(target_get_tailcall_unwinder): Rewrite.
	* record-btrace.c (record_btrace_to_get_unwinder): New function.
	(record_btrace_to_get_tailcall_unwinder): New function.
	(init_record_btrace_ops): Update.
	* target.h (struct target_ops) <to_get_unwinder,
	to_get_tailcall_unwinder>: Now function pointers.  Use
	TARGET_DEFAULT_RETURN.