arm-dis.c (print_insn_coprocessor): Rename index to index_operand.

Older gcc warns, arguably incorrectly, about name collisions between
global functions and function-local variable names.  Consesus has been
to rename local variables whenever this is spotted, hence committed as
obvious.  Note the pre-existing variable named idx; "index_operand"
seemed logical given the context.

	* arm-dis.c (print_insn_coprocessor): Rename index to
	index_operand.