include/sys/cdefs.h: Add __attribute_optimization_barrier__

Add __attribute_optimization_barrier__ to disable inlining and cloning on a
function.  For Clang, expand it to

__attribute__ ((optnone))

Otherwise, expand it to

__attribute__ ((noinline, clone))

Co-Authored-By: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>

malloc/tst-malloc-thread-exit.c

@@ -42,7 +42,7 @@ static int inner_thread_count = 4;
 static size_t malloc_size = 32;
 
 static void
-__attribute__ ((noinline, noclone))
+__attribute_optimization_barrier__
 unoptimized_free (void *ptr)
 {
   free (ptr);