Merge pull request #444 from RISCVERS/fix-mstatus-reset

backend,fu,CSR: clear mstatus.MPP upon reset

src/main/scala/xiangshan/backend/fu/CSR.scala

@@ -236,7 +236,7 @@ class CSR extends FunctionUnit with HasCSRConst
   val mimpid = RegInit(UInt(XLEN.W), 0.U) // provides a unique encoding of the version of the processor implementation
   val mhartNo = hartId()
   val mhartid = RegInit(UInt(XLEN.W), mhartNo.asUInt) // the hardware thread running the code
-  val mstatus = RegInit(UInt(XLEN.W), "h00001800".U)  // another option: "h8000c0100".U
+  val mstatus = RegInit(UInt(XLEN.W), 0.U)
 
   // mstatus Value Table
   // | sd   |