csrrw mcause discrepancy between gem5 and nemu

[HaoyuanLiuHZPH]

Describe the bug
When executing the instruction csrrw zero, mstatus, sx, a discrepancy is observed in the XS[1:0], MBE, and SBE fields of the mcause CSR between XIANGSHAN NEMU and GEM5. In XIANGSHAN NEMU, these fields are read only zero, whereas in GEM5, they are not.
To Reproduce
bin file path:/nfs/home/liuhaoyuan/GEM5/img_file/seeds_999_.img or any bin file contains "csrrw zero, mstatus, sx"

Expected behavior
difftest fail

Necessary information on versions

• XS-GEM5 version: [newest ]
• NEMU version used as reference design: ["/nfs/home/share/gem5_ci/ref/normal/riscv64-nemu-interpreter-so"]

[jueshiwenli]

It may be caused by a mask mismatch. You can first check whether the mask configuration in mstatus is consistent.

[jensen-yan]

fixed in #759