sw64: fix incorrect gp after kretprobe triggered

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5XTM4



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

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