Fix minor errors with handling Opella targets.

	* arc-tdep.c (arc_find_this_sp): Set tdep after debug message.
	(arc_scan_prologue): Set tdep after gdbarch is set.
	(arc_register_name): Remove "pc" from oa6 and oa7 lists.
	(arc_gdbarch_init): Use AUX_PC with OA6 and OA7 targets.
	* arc-tdep.c (OA6_PC, OA7_PC): Remove and renumber remaining
	registers.