Fix __attribute__ (( __attribute__ ((noreturn)))), which doesn't compile.

include/unistd.h

@@ -771,7 +771,7 @@ libc_hidden_proto(vfork)
 #endif /* Use BSD. */
 
 /* Special exit function which only terminates the current thread.  */
-extern void __exit_thread (int val) __attribute__ ((noreturn));
+extern void __exit_thread (int val) __attribute__ ((__noreturn__));
 
 /* Return the pathname of the terminal FD is open on, or NULL on errors.
    The returned storage is good only until the next call to this function.  */