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: Cyrill Gorcunov <gorcunov@gmail.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>