target/riscv: rvv-1.0: add vlenb register

target/riscv: rvv-1.0: add vlenb register

Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-11-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>