sw64: reimplement get_wchan()

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5GFUD



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The get_wchan() always return 0 because `fp > sp` is false. This
patch reimplements it entirely and fixes this error.

Signed-off-by: Wu Liliu <wuliliu@wxiat.com>

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