opcodes/arc: Move instruction length logic to new function

Move the logic that calculates the instruction length out to a new
function.  Restructure the code to make it simpler.

opcodes/ChangeLog:

	* arc-dis.c (arc_insn_length): New function.
	(print_insn_arc): Use arc_insn_length, change insnLen to unsigned.
	(find_format): Change insnLen parameter to unsigned.