No Exception Raised for Accessing Non-Existent CSR in Machine Mode

[fly-1011]

Bug Description:

When accessing a non-existent CSR (e.g., 0xb6e) in M ​​mode, NutShell does not raise an exception.

Examining the CSR in the NutShell source code (https://github.com/OSCPU/NutShell/blob/61923a1fe947354ef1674ddc4df5e1c6f299ce97/src/main/scala/nutcore/backend/fu/CSR.scala), no custom implementation for this CSR was found.

Inconsistent information:

Test program and log information: test_unknown.zip

Environment:

NutShell: commit e315a27