sw64: remove switch_stack from __sw64_vcpu_run

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



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Since r9~r15 have been added to struct pt_regs, it's okay to save
and restore host callee-saved integer registers with pt_regs.

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

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