пример работает

libraries/SoftwareSerial/src/SoftwareSerial.cpp

@@ -145,8 +145,7 @@ void SoftwareSerial::handle_interrupt() // .ram_text
   }
 }
 
-extern "C" void softSerial_interrupt_handler(void) __attribute__((noinline, section(".ram_text")));
-extern "C" void softSerial_interrupt_handler(void)
+extern "C" void __attribute__((noinline, section(".ram_text"))) softSerial_interrupt_handler(void)
 {
   SoftwareSerial::handle_interrupt();
 }