fix(mstatus.mpp): 写入非法值时，mpp保持原值

更新逻辑修改为当往mpp中写入CPU未实现的状态时，mpp保持原值不变

difftest/src/rv_priv.hpp

@@ -267,7 +267,7 @@ public:
                 assert(mstatus->spie != 2);
                 assert(mstatus->mpie != 2);
                 // mstatus->spp = nstatus->spp;
-                mstatus->mpp = (nstatus->mpp == 3 || nstatus->mpp == 0) ? nstatus->mpp : 0;
+                mstatus->mpp = (nstatus->mpp == 3 || nstatus->mpp == 0) ? nstatus->mpp : mstatus->mpp;
                 mstatus->mprv = nstatus->mprv;
                 // mstatus->sum = nstatus->sum; // always true
                 // mstatus->mxr = nstatus->mxr; // always true