sched/idle: Make sure weak version of arch_cpu_idle_dead() doesn't return

arch_cpu_idle_dead() should never return.  Make it so.

Link: https://lore.kernel.org/r/cf5ad95eef50f7704bb30e7770c59bfe23372af7.1676358308.git.jpoimboe@kernel.org


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