arc: Replace ARC_SHORT macro with arc_opcode_len function

In preparation for moving to a world where arc instructions can be 2, 4,
6, or 8 bytes in length, replace the ARC_SHORT macro (which is either
true of false) with an arc_opcode_len function that returns a length in
bytes.

There should be no functional change after this commit.

gas/ChangeLog:

	* config/tc-arc.c (assemble_insn): Replace use of ARC_SHORT with
	arc_opcode_len.

include/ChangeLog:

	* opcode/arc.h (arc_opcode_len): Declare.
	(ARC_SHORT): Delete.

opcodes/ChangeLog:

	* arc-dis.c (find_format_from_table): Replace use of ARC_SHORT
	with arc_opcode_len.
	(find_format_long_instructions): Likewise.
	* arc-opc.c (arc_opcode_len): New function.