By the manual of LoongArch CSR, the VS field(18:16 bits) of
ECFG reg means that the number of instructions between each
exception entry is 2^VS.

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220701093407.2150607-9-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>