arc: Swap highbyte and lowbyte in print_insn_arc

highbyte and lowbyte actually refer to the low byte and the high
byte respectively, but are used consistently in this swapped
order. This commit swaps them round so that highbyte refers to the
high byte and lowbyte refers to the low byte.

There should be no functional change after this commit.

opcodes/ChangeLog:

   * arc-dis.c (print_insn_arc): Swap highbyte and lowbyte.