x86: lds - Use THREAD_SIZE instead of numeric constant

Though we use PDA for regular task stack but that
is not acceptable for init_task wich is special
one. We still have to allocate init_task's stack
in that manner.
Signed-off-by: NCyrill Gorcunov <gorcunov@gmail.com>
Acked-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/vmlinux_64.lds.S

@@ -129,7 +129,7 @@ SECTIONS
 #undef VVIRT_OFFSET
 #undef VVIRT
 
-  . = ALIGN(8192);		/* init_task */
+  . = ALIGN(THREAD_SIZE);	/* init_task */
   .data.init_task : AT(ADDR(.data.init_task) - LOAD_OFFSET) {
 	*(.data.init_task)
   }:data.init