The SIGNAL() is a macro to define a function to register to interrupt vectors.

The SIGNAL() is defined in hardware/tools/avr/avr/include/avr/interrupt.h as below.

#  define __INTR_ATTRS used, externally_visible

#  define SIGNAL(vector)                                        \
    extern "C" void vector(void) __attribute__ ((signal, __INTR_ATTRS));        \
    void vector (void)

The 'signal' in the __attribute__ means that it is an interupt handler. The 'used' tells the compiler to write code to a file unless it looks unused. The 'externally_visible' invalidates the effect of the '-fwhole_program', which is a gcc compiler option.