Incorrect VS field handling in mstatus register

[fly-1011]

Bug Description:

NutShell does not support the V extension. According to the RISC-V Privileged Specification, when the vector extension is not implemented, the VS field in the mstatus register should be 00. However, in the NutShell implementation, the VS field is incorrectly set to a non-zero value.

Screenshots

Expected Behavior:

The VS field should always be 00 when the vector extension is not enabled.

Actual Behavior:

The VS field in the mstatus register is incorrectly set to 10. The differences in other bits of the register are mentioned in #207