Sim800L's library can not compile on Arduino Due
First we must delete the SoftwareSerial and SIM800L libraries, then define Sim800l and assign it to Serial 3 on pins 14 and 15. In setup, call Sim800l.begin() and the rest of the code is unchanged.

#define SIM800l Serial3

void setup () {
  Serial.begin (9600);
  SIM800l.begin (9600);
  while(!Serial);
  SIM800l.write ("AT+CMGF=1\r\n");
  delay (1000);
  SIM800l.write("AT+CMGS=\"5432154552"\"\r\n");
  delay(1000);
  Serial.println("send SMS");
  SIM800l.write ("test send SMS");
  delay (1000);
  SIM800l.write ((char)26);
  delay(1000);
}

void loop (){}