Fix argument passing for call

Fix argument passing for call

When calling function with argument of size more than 8 bytes fails with
an error "That operation is not available on integers of more than 8 bytes.".
avr-gdb considers only 8 bytes (sizeof(long long)) in case of passing the
argument in registers. When the argument is of size more than 8 byte
then the utility function to extract bytes failed with the above error.

    gdb/
	* avr-tdep.c (AVR_LAST_ARG_REGNUM): Define.
        (avr_push_dummy_call): Correct last needed argument register.
        Write MSB of argument into register and subsequent bytes into
        other registers in decreasing order.