ck_cc: Add CK_CC_BARRIER for compiler barrier.

include/ck_cc.h

@@ -45,6 +45,10 @@
 
 #define CK_CC_PAD(x) union { char pad[x]; }
 
+#ifndef CK_CC_BARRIER
+#define CK_CC_BARRIER()
+#endif
+
 #ifndef CK_CC_ALIASED
 #define CK_CC_ALIASED
 #endif

include/gcc/ck_cc.h

@@ -68,6 +68,11 @@
  */
 #define CK_CC_CACHELINE CK_CC_ALIGN(CK_MD_CACHELINE)
 
+/*
+ * Compiler barrier.
+ */
+#define CK_CC_BARRIER() __asm__ __volatile__("" ::: "memory")
+
 /*
  * These are functions which should be avoided.
  */