target/riscv: Add standard B extension implied rule

Add the missing implied rule for standard B extension. Standard B extension implies Zba, Zbb, Zbs extensions. We can also remove the auto-enables in riscv_cpu_validate_b() as Zba, Zbb, Zbs extensions can be enabled by the implied rule. RISC-V B spec: https://github.com/riscv/riscv-b Reviewed-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com> Reviewed-by: Jim Shu <jim.shu@sifive.com> Reviewed-by: Alistair Francis <alistair.francis@wdc.com> Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com> Signed-off-by: Frank Chang <frank.chang@sifive.com> Signed-off-by: Frank Chang <frank.chang@sifive.com> Message-ID: <20260528054213.678458-3-frank.chang@sifive.com> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>