Serial Transmit on Due stops working after using pinMode

Hi,
Tx for Serial3 for Arduino Due doesn't work after using pinMode on the pin. No issue is there for receiving. I only faced this issue on Arduino Due, not on Mega or MKR boards. Please see the reproduction code below:

I have TX3 and RX3 shorted on the Due.

```
void setup()
{
Serial.begin(9600);
pinMode(14,OUTPUT); //14 is TX3
Serial3.begin(9600);
}

void loop()
{
Serial3.write(65);
delay(1000);
if(Serial3.available()>0)
{
Serial.println(Serial3.read());
}
else
{
Serial.println("No Data");
}
}
```
Serial Monitor displays only "No Data". Now comment the line
`pinMode(14,OUTPUT); //14 is TX3`

The serial monitor starts printing 65 as expected.

I am using Arduino 1.8.8.

Please let me know if there is something I am missing.

Thank you
Regards
Gayatri

Start by running the supplied sketch on an Arduino Due with TX3 and RX3 shorted, comparing behavior with and without pinMode(14, OUTPUT). Trace the Due-specific pinMode and Serial3 initialization paths; done means Serial3.write(65) is received and printed after pinMode is called, without breaking the existing receiving behavior.

Written by the indexing model from the issue text.