target/riscv/cpu.c: introduce cpu_cfg_ext_auto_update()

target/riscv/cpu.c: introduce cpu_cfg_ext_auto_update()

During realize() time we're activating a lot of extensions based on some
criteria, e.g.:

    if (cpu->cfg.ext_zk) {
        cpu->cfg.ext_zkn = true;
        cpu->cfg.ext_zkr = true;
        cpu->cfg.ext_zkt = true;
    }

This practice resulted in at least one case where we ended up enabling
something we shouldn't: RVC enabling zca/zcd/zcf when using a CPU that
has priv_spec older than 1.12.0.

We're also not considering user choice. There's no way of doing it now
but this is about to change in the next few patches.

cpu_cfg_ext_auto_update() will check for priv version mismatches before
enabling extensions. If we have a mismatch between the current priv
version and the extension we want to enable, do not enable it. In the
near future, this same function will also consider user choice when
deciding if we're going to enable/disable an extension or not.

For now let's use it to handle zca/zcd/zcf enablement if RVC is enabled.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20230912132423.268494-16-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>