gdb: Extend the trad-frame API

gdb: Extend the trad-frame API

Adds two new functions to the trad-frame API and update the internals
of trad-frame to use the new functions.  These functions will be used
in later commits.

gdb/ChangeLog:

	* trad-frame.h (trad_frame_set_realreg): Declare.
	(trad_frame_set_addr): Declare.
	* trad-frame.c (trad_frame_set_realreg): Define new function.
	(trad_frame_set_addr): Define new function.
	(trad_frame_set_reg_realreg): Use new function.
	(trad_frame_set_reg_addr): Use new function.