target/riscv: Fix vill field write in vtype

The guest should be able to set the vill bit as part of vsetvl.

Currently we may set env->vill to 1 in the vsetvl helper, but there
is nowhere that we set it to 0, so once it transitions to 1 it's stuck
there until the system is reset.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220201064601.41143-1-zhiwei_liu@c-sky.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>