target/riscv: remove cpu->cfg.ext_u

target/riscv: remove cpu->cfg.ext_u

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

Remove the old "u" property and 'ext_u' 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: <20230406180351.570807-14-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>