target/riscv: make riscv_cpu_is_vendor() public

target/riscv: make riscv_cpu_is_vendor() public

We'll use this function in target/riscv/cpu.c to implement setters that
won't allow vendor CPU options to be changed.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Vladimir Isaev <vladimir.isaev@syntacore.com>
Message-ID: <20240105230546.265053-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>