We need subcodes to distinguish the same excode cs->exception_indexs,
such as EXCCODE_ADEF/EXCCODE_ADEM.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20221101073210.3934280-1-gaosong@loongson.cn>