rx.c (rx_conditional_register_usage): Do not warn if no fixed registers are available for interrupt handlers.

        * config/rx/rx.c (rx_conditional_register_usage): Do not warn if
        no fixed registers are available for interrupt handlers.  Do not
        fix normal registers.
        (MUST_SAVE_ACC_REGISTER): New macro.
        (rx_get_stack_layout): Create a stack frame for fast interrupt
        handlers, if necessary.  Push extra registers if needed in order
        to save the accumulator.
        (rx_expand_prologue): Create a stack frame for fast interrupt
        handlers, if necessary.
        (rx_expand_builtin_mvfc): Handle the case where there is no
        target.

From-SVN: r154903