Add standalone memory barrier function to project.

memory_barrier.h

@@ -0,0 +1,18 @@
+#ifndef _MEMORY_BARRIER_H_
+#define _MEMORY_BARRIER_H_
+
+#include <util/atomic.h>
+#include <avr/version.h>
+
+// Provide a memory barrier to the compiler. This informs
+// the compiler that is should write any cached values that
+// are destined for a global variable and discard any other
+// cached values from global variables.
+//
+// Note that this behavior does apply to all global variables,
+// not just volatile ones. However, cached local variables
+// are not affected as they are not externally visible.
+
+#define MEMORY_BARRIER() __asm volatile( "" ::: "memory" )
+
+#endif