hexagon: Use generic idle_task

Same code. Use the generic version. The special Makefile treatment is
pointless anyway as init_task.o contains only data which is handled by
the linker script. So no point on being treated like head text.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20120503085034.692078846@linutronix.de
Acked-and-tested-by: Richard Kuo <rkuo@codeaurora.org>