hexagon: Use common threadinfo allocator

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

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Kuo <rkuo@codeaurora.org>
Link: http://lkml.kernel.org/r/20120505150141.812612113@linutronix.de