sw64: fix exception handling of deep-copy_user.S

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PN9S



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If exception happened inside simd part causing the function to exit,
pop stack to make sure everything works correctly.

Signed-off-by: Mao Minkai <maominkai@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>