sw64: split out the idle loop in idle.c

Sunway inclusion
category: performance
bugzilla: https://gitee.com/openeuler/kernel/issues/I5XTK7



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Signed-off-by: Dai Xin <daixin@wxiat.com>
Reviewed-by: He Sheng <hesheng@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>