arm/cpu: Add unreachable() to arch_cpu_idle_dead()

arch_cpu_idle_dead() doesn't return.  Make that visible to the compiler
with an unreachable() code annotation.

Link: https://lkml.kernel.org/r/20230216183851.s5bnvniomq44rytu@treble


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