target/riscv/cpu.c: add riscv_cpu_validate_v()

target/riscv/cpu.c: add riscv_cpu_validate_v()

The RVV verification will error out if fails and it's being done at the
end of riscv_cpu_validate_set_extensions(), after we've already set some
extensions that are dependent on RVV.  Let's put it in its own function
and do it earlier.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230517135714.211809-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>