sw64: fix instruction fault handler

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PNCX



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An issue says that when parent traces a child, child does not terminate
after signal SIGILL is delivered. This is because BPT/GENTRAP/OPDEC make
regs->pc = exc_pc + 4 in hmcode. It used to send SIGILL to child with
incorrect regs->pc. As a result, after PTRACE_CONT request is made on
child, it will skip the exception instruction and go on.

This patch makes `regs->pc = exc_pc` for SIGILL to fix this issue, then
restructures code to make it clear.

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