Fix STM32 flush of TX (used by UBL) (#16197)

Marlin/src/core/serial.h

 #define SERIAL_PRINTF(V...)     SERIAL_OUT(printf, V)
 #define SERIAL_FLUSH()          SERIAL_OUT(flush)
 
-#ifdef __STM32F1__
+#ifdef ARDUINO_ARCH_STM32
   #define SERIAL_FLUSHTX()      SERIAL_OUT(flush)
 #elif TX_BUFFER_SIZE > 0
   #define SERIAL_FLUSHTX()      SERIAL_OUT(flushTX)