sw64: remove switch_stack and allregs from entUna

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



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It's simple to maintain unified register layout for user-mode and
kernel-mode unaligned accesses. With the new struct pt_regs, we
are able to make the exception handler cleaner.

After that, the unused struct allregs can be cleaned up.

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

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