target/riscv: add RVG and remove cpu->cfg.ext_g

target/riscv: add RVG and remove cpu->cfg.ext_g

We're still have one RISCVCPUConfig MISA flag, 'ext_g'. We'll remove it
the same way we did with the others: create a "g" RISCVCPUMisaExtConfig
property, remove the old "g" property, remove all instances of 'cfg.ext_g'
and use riscv_has_ext(env, RVG).

The caveat is that we don't have RVG, so add it. RVG will be used right
off the bat in set_misa() of rv64_thead_c906_cpu_init() because the CPU is
enabling G via the now removed 'ext_g' flag.

After this patch, there are no more MISA extensions represented by flags
in 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: <20230406180351.570807-20-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>