target/riscv: remove cpu->cfg.ext_a

target/riscv: remove cpu->cfg.ext_a

Create a new "a" RISCVCPUMisaExtConfig property that will update
env->misa_ext* with RVA. Instances of cpu->cfg.ext_a and similar are
replaced with riscv_has_ext(env, RVA).

Remove the old "a" property and 'ext_a' from RISCVCPUConfig.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230406180351.570807-6-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>