target/riscv: support vector extension csr

target/riscv: support vector extension csr

The v0.7.1 specification does not define vector status within mstatus.
A future revision will define the privileged portion of the vector status.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200701152549.1218-4-zhiwei_liu@c-sky.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>