sw64: reimplement show_stack() method

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5GFQ0



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It used to show stack information by SP, that means it has nothing to
do with task, which is unexpected. In order to show stack of task, we
improve the implementation of show_stack().

However, the original task struct does not have sp, so we add it into
thread_struct and do something necessary in __switch_to.

Meanwhile, walk_stackframe() can be reused, so refactor related codes
for better support.

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

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