gas/arc: Replace short_insn flag with insn length field

When assembling an instruction replace the short_insn boolean flag with
an integer field for holding the instruction length.  This is in
preparation for moving to a world where instructions can be 2, 4, 6, or
8 bytes in length.

gas/ChangeLog:

	* config/tc-arc.c (struct arc_insn): Replace short_insn flag with
	len field.
	(apply_fixups): Update to use len field.
	(emit_insn0): Simplify code, making use of len field.
	(md_convert_frag): Update to use len field.
	(assemble_insn): Update to use len field.