Define a _NOP() macro

Recent avr-libc releases define one, but this allows using it also on
older avr-libc releases.

Author: matthijskooijman

hardware/arduino/avr/cores/arduino/Arduino.h

@@ -88,6 +88,10 @@ void yield(void);
 #define bitClear(value, bit) ((value) &= ~(1UL << (bit)))
 #define bitWrite(value, bit, bitvalue) (bitvalue ? bitSet(value, bit) : bitClear(value, bit))
 
+// avr-libc defines _NOP() since 1.6.2
+#ifndef _NOP
+#define _NOP() do { __asm__ volatile ("nop"); } while (0)
+#endif
 
 typedef unsigned int word;