HAL_SITL: fixed unbuffered UART writes in SITL

this was causing PPP to slow down by about 1000x due to duplicate
writes

libraries/AP_HAL_SITL/UARTDriver.cpp

@@ -787,6 +787,7 @@ uint16_t UARTDriver::read_from_async_csv(uint8_t *buffer, uint16_t space)
 
 void UARTDriver::handle_writing_from_writebuffer_to_device()
 {
+    WITH_SEMAPHORE(write_mtx);
     if (!_connected) {
         _check_reconnect();
         return;

libraries/AP_HAL_SITL/UARTDriver.h

@@ -110,6 +110,8 @@ private:
     uint32_t last_read_tick_us;
     uint32_t last_write_tick_us;
 
+    HAL_Semaphore write_mtx;
+
     SITL::SerialDevice *_sim_serial_device;
 
     struct {