sh-use-common-threadinfo-allocator

The core now has a threadinfo allocator which uses a kmemcache when
THREAD_SIZE < PAGE_SIZE.

Deal with the xstate cleanup in the new arch_release_task_struct()
function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul Mundt <lethal@linux-sh.org>
Link: http://lkml.kernel.org/r/20120505150142.189348931@linutronix.de