Create macro to guard critical sections for large transmit buffers

New macro TX_BUFFER_ATOMIC makes the following code block atomic
only if the transmit buffer is larger than 256 bytes. SREG is restored
on completion.
The macro is then used to simplify code for availableForWrite()

cores/arduino/HardwareSerial.cpp

@@ -26,6 +26,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <inttypes.h>
+#include <util/atomic.h>
 #include "Arduino.h"
 
 #include "HardwareSerial.h"
@@ -76,6 +77,13 @@ void serialEventRun(void)
 #endif
 }
 
+// macro to guard critical sections when needed for large TX buffer sizes
+#if (SERIAL_TX_BUFFER_SIZE>256)
+#define TX_BUFFER_ATOMIC ATOMIC_BLOCK(ATOMIC_RESTORESTATE)
+#else
+#define TX_BUFFER_ATOMIC
+#endif
+
 // Actual interrupt handlers //////////////////////////////////////////////////////////////
 
 void HardwareSerial::_tx_udr_empty_irq(void)
@@ -177,15 +185,13 @@ int HardwareSerial::read(void)
 
 int HardwareSerial::availableForWrite(void)
 {
-#if (SERIAL_TX_BUFFER_SIZE>256)
-  uint8_t oldSREG = SREG;
-  cli();
-#endif
-  tx_buffer_index_t head = _tx_buffer_head;
-  tx_buffer_index_t tail = _tx_buffer_tail;
-#if (SERIAL_TX_BUFFER_SIZE>256)
-  SREG = oldSREG;
-#endif
+  tx_buffer_index_t head;
+  tx_buffer_index_t tail;
+
+  TX_BUFFER_ATOMIC {
+    head = _tx_buffer_head;
+    tail = _tx_buffer_tail;
+  }
   if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
   return tail - head - 1;
 }