Split out constant declarations, so can be reused by gdbserver. Assorted code
improvements.

2013-02-18  Jeremy Bennett <jeremy.bennett@embecosm.com>

	* arc-aux-registers.h (_initialize_arc_aux_regs): Replace
	ARC_VariantsInfo by underlying struct definition.
	* arc-constants.h: Created.
	* arc-elf-tdep.c (create_variant_info): Replace ARC_VariantsInfo
	by underlying struct definition.
	* arc-tdep.h: Break out constants into arc-constants.h
	(ARC_VariantsInfo): Remove typedef
	(gdbarch_tdep): Replace ARC_VariantsInfo by underlying struct
	definition.