arch_cpu_idle_dead() doesn't return.  Make that more explicit with a
BUG().

BUG() is preferable to unreachable() because BUG() is a more explicit
failure mode and avoids undefined behavior like falling off the edge of
the function into whatever code happens to be next.
Acked-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/1e9ecc3d248e82973e80bc336fc9f97e3ba2708d.1676358308.git.jpoimboe@kernel.orgSigned-off-by: Josh Poimboeuf <jpoimboe@kernel.org>