xtensa/cpu: Mark cpu_die() __noreturn

cpu_die() doesn't return.  Annotate it as such.  By extension this also
makes arch_cpu_idle_dead() noreturn.

Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Link: https://lore.kernel.org/r/ad801544cab7c26a0f3bbf7cfefb67303f4cd866.1676358308.git.jpoimboe@kernel.org


Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>