Blackfin: fix the section name of init_thread_union

Use the common attribute rather than setting the section name directly.
The common linker script defines expect the newer naming.

Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/blackfin/kernel/init_task.c

@@ -28,5 +28,5 @@ EXPORT_SYMBOL(init_task);
  * "init_task" linker map entry.
  */
 union thread_union init_thread_union
-    __attribute__ ((__section__(".init_task.data"))) = {
+    __init_task_data = {
 INIT_THREAD_INFO(init_task)};