Fix compiler_memory_barrier

Especially on armhf, clobbering "cc" does matter for preventing the
compiler from holding information derived from information in memory
across the barrier.

Change-Id: I468c5a3596af237f1424ba72cb0262cf7cdf5553
diff --git a/aos/util/compiler_memory_barrier.h b/aos/util/compiler_memory_barrier.h
index 33da511..6be3c59 100644
--- a/aos/util/compiler_memory_barrier.h
+++ b/aos/util/compiler_memory_barrier.h
@@ -5,7 +5,7 @@
 // Using this function makes it clearer what you're doing and easier to be
 // portable.
 static inline void aos_compiler_memory_barrier(void) {
-  __asm__ __volatile__("" ::: "memory");
+  __asm__ __volatile__("" ::: "memory", "cc");
 }
 
 #endif  // AOS_UTIL_COMPILER_MEMORY_BARRIER_H_