sw64: get blocked thread's frame pointer from thread_struct

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5GF7A



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The thread_struct holds all provided the thread blocked through
a call to schedule(). $15 is the frame pointer in schedule() and
it is saved at thread_struct.s[6]. With frame pointer, it can
return saved PC of a blocked thread which assumes that the saved
return address is the first long in the frame.

Signed-off-by: He Sheng <hesheng@wxiat.com>

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