sw64: fix incorrect gp after uretprobe triggered

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56OLG



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SW64 use r26 to calculate gp after function return, so r26 needs to be
restored when uretprobe trampoline is hit.

Since uretprobe is handled in generic code, we will modify r26 before we
return to user space.

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