libroot: Some fixes for Clang ARM builds.

src/system/libroot/os/arch/arm/thread.c

@@ -21,6 +21,7 @@ find_thread(const char *name)
 }
 
 
+#if !defined(__clang__)
 /*
  * Fill out gcc __sync_synchronize built-in for ARM
  */
@@ -29,3 +30,4 @@ __sync_synchronize(void)
 {
 	dmb();
 }
+#endif

src/system/libroot/os/atomic.c

@@ -108,7 +108,7 @@ atomic_get64(int64 *value)
 
 #endif	/* ATOMIC64_FUNCS_ARE_SYSCALLS */
 
-#if defined(__arm__)
+#if defined(__arm__) && !defined(__clang__)
 
 /* GCC compatibility: libstdc++ needs this one.
  * TODO: Update libstdc++ and drop this.