target/riscv: smstateen knobs

target/riscv: smstateen knobs

Add knobs to allow users to enable smstateen and also export it via the
ISA extension string.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Weiwei Li<liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230518175058.2772506-4-mchitale@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>