sw64: signal: save/restore fpregs with copy user

Sunway inclusion
category: performance
bugzilla: https://gitee.com/openeuler/kernel/issues/I56XPR



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Both context_fpregs and sc_fpregs are contigous, so it may improve
performance with copy_{from,to}_user.

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

Signed-off-by: Gu Zitao <guzitao@wxiat.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>