Add options to handle Opella targets.

	* arc-tdep.c (setarccmdlist, showarccmdlist, arc_opella_string):
	Static data created.
	(arc_gdbarch_init): Set register data in target dependencies,
	based on opella setting. Use when initializing architecture.
	(arc_dump_tdep): Implementation added.
	(set_arc_command, show_arc_command, arc_set_opella): Created.
	(initialize_arc_tdep): Add "set/show arc opella-target" command.
	* arc-tdep.h: Add Opella ARC700 register numbering.
	(arc_opella_target): New enumeration.
	(gdbarch_tdep): Add opella target and register numbers.
Use target specific register numbers.

	* arc-elf-tdep.c (arc_elf_cannot_fetch_register)
	(arc_elf_cannot_store_register): Make dependent on Opella target.
	* arc-tdep.c (arc_print_frame_info, arc_frame_base_address)
	(arc_find_this_sp, arc_is_callee_saved)
	(arc_is_in_prologue, arc_scan_prologue, arc_virtual_frame_pointer)
	(arc_push_dummy_call, arc_print_registers_info)
	(arc_get_longjump_target, arc_skip_trampoline_code)
	(arc_frame_this_id, arc_frame_prev_register)
	(arc_dwarf2_frame_init_reg, arc_sigtramp_frame_this_id): Use
	register numbers from target dependency structure.
	(arc_register_name): Make result dependent on Opella target.
	(arc_register_type): Make results dependent on Opella target, use
	register numbers from target dependency structure.
	(arc_gdbarch_init): Set up common target dependency fields, add
	number of core regs to tdep.
	(arc_dump_tdep): Extend for new fields.
	* arc-tdep.h: Add Opella ARC600 definitions. Define target
	specific number of core regs.
	(ARC_TOTAL_REGS): Deleted.
	(gdbarch_tdep): Add fields for number of core regs and first/last
	arg and callee-saved regs.
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.
Better help text for set/show arc opella-target

	* arc-tdep.c (_initialize_arc_tdep): Clearer help text for set arc
	opella-target.
Documentation updated for set/show arc opella-target

	* doc/gdb.texinfo (Synopsys ARC600, ARC700 and ARC v2 EM): Added
	details of set/show arc opella-target and the registers supported
	by this.