sw64: remove switch_stack from entMM and entSys

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



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This patch removes switch_stack and its usages in mm fault entry
and system call entry. Note that ptrace accesses registers with
struct pt_regs only since strace gave up a switch_stack trick.

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

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