sw64: improve stack trace

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I645NW



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Process ra directly if call stack has not been setup yet so we won't
miss the previous function.

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