sw64: guard hotplug cpu specific code

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56OLG



--------------------------------

Add #ifdef guard around hotplug cpu specific code.

It also moves arch_cpu_idle_dead into smp.c because HOTPLUG_CPU
depends on SMP.

Signed-off-by: He Sheng <hesheng@wxiat.com>
Reviewed-by: Cui Wei <cuiwei@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>