sw64: init sp with kernel stack top for idle thread

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PNF1



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For secondary CPU, it has to do ($sp - THREAD_SIZE) to find current
thread_info.

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