target/riscv: rvv-1.0: introduce writable misa.v field

target/riscv: rvv-1.0: introduce writable misa.v field

Implementations may have a writable misa.v field. Analogous to the way
in which the floating-point unit is handled, the mstatus.vs field may
exist even if misa.v is clear.

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: <20211210075704.23951-7-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>