target/riscv: Not allow write mstatus_vs without RVV

target/riscv: Not allow write mstatus_vs without RVV

If CPU does not implement the Vector extension, it usually means
mstatus vs hardwire to zero. So we should not allow write a
non-zero value to this field.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231215023313.1708-1-zhiwei_liu@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>