target/riscv: Mask the implicitly enabled extensions in isa_string based on priv...

target/riscv: Mask the implicitly enabled extensions in isa_string based on priv version

Using implicitly enabled extensions such as Zca/Zcf/Zcd instead of their
super extensions can simplify the extension related check. However, they
may have higher priv version than their super extensions. So we should mask
them in the isa_string based on priv version to make them invisible to user
if the specified priv version is lower than their minimal priv version.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230517135714.211809-6-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>