Generalize OSABI identification, move externs to headers.

2013-01-31  Jeremy Bennett <jeremy.bennett@embecosm.com>

	* arc-linux-tdep.c (arcAnalzerInstr, arc_debug): Move extern
	declarations to header.
	(arc_get_osabi): Created.
	* arc-tdep.c: Minor tidying up of comments and layout,
	(arc_gdb_init): Set info.osabi using arc_get_osabi.
	* arc-tdep.h (arcAnalzerInstr, arc_debug, arc_get_osabi): Add
	extern declarations.