sw64: perf: fix perf_get_regs_user

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I56X48



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The past implementation of perf_get_regs_user could not get regs by
default, but it was actually available via task_pt_regs. Fix it now.

Signed-off-by: He Chuyue <hechuyue@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>