Mark SoftwareSerial::tx_pin_write as "always_inline"

Somehow gcc 4.8 doesn't inline this function, even though it is always
called with constant arguments and can be reduced to just a few
instructions when inlined. Adding the always_inline attribute makes gcc
inline it, saving 46 bytes on the Arduino uno.

gcc 4.3 already inlined this function, so there are no space
savings there.

hardware/arduino/avr/libraries/SoftwareSerial/SoftwareSerial.h

@@ -71,7 +71,7 @@ private:
   // private methods
   void recv();
   uint8_t rx_pin_read();
-  void tx_pin_write(uint8_t pin_state);
+  void tx_pin_write(uint8_t pin_state) __attribute__((__always_inline__));
   void setTX(uint8_t transmitPin);
   void setRX(uint8_t receivePin);
   void setRxIntMsk(bool enable);