Fix Hardwareserial sending a byte twice

https://github.com/arduino/Arduino/issues/3745

Author: NicoHood

hardware/arduino/avr/cores/arduino/HardwareSerial.cpp

@@ -218,8 +218,11 @@ size_t HardwareSerial::write(uint8_t c)
   // significantly improve the effective datarate at high (>
   // 500kbit/s) bitrates, where interrupt overhead becomes a slowdown.
   if (_tx_buffer_head == _tx_buffer_tail && bit_is_set(*_ucsra, UDRE0)) {
+    uint8_t oldSREG = SREG;
+    cli();
     *_udr = c;
     sbi(*_ucsra, TXC0);
+    SREG = oldSREG;
     return 1;
   }
   tx_buffer_index_t i = (_tx_buffer_head + 1) % SERIAL_TX_BUFFER_SIZE;
@@ -240,9 +243,12 @@ size_t HardwareSerial::write(uint8_t c)
   }
 
   _tx_buffer[_tx_buffer_head] = c;
+  uint8_t oldSREG = SREG;
+  cli();
   _tx_buffer_head = i;
 
   sbi(*_ucsrb, UDRIE0);
+  SREG = oldSREG;
 
   return 1;
 }