sw64: remove switch_stack from signal handling

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



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The new pt_regs struct holds r9 ~ r15, and it's sufficient to setup
and restore sigcontext. Take care that fpregs state of current task
should be saved and restored explicitly after switch_stack removed.

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

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