target/riscv: Introduce extension implied rules definition

target/riscv: Introduce extension implied rules definition

RISCVCPUImpliedExtsRule is created to store the implied rules.
'is_misa' flag is used to distinguish whether the rule is derived
from the MISA or other extensions.
'ext' stores the MISA bit if 'is_misa' is true. Otherwise, it stores
the offset of the extension defined in RISCVCPUConfig. 'ext' will also
serve as the key of the hash tables to look up the rule in the following
commit.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com>
Tested-by: Max Chou <max.chou@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240625114629.27793-2-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>