i960.c (i960_function_arg_advance): Ensure all regs marked as used if stack is also used (for va_start).

	* i960/i960.c (i960_function_arg_advance): Ensure all regs marked
	as used if stack is also used (for va_start).
	(i960_setup_incoming_varargs): Rewrite to be similar to Intel's
	version, but don't allocate reg block unless necessary.

From-SVN: r9226