target/riscv: Add additional xlen for address when MPRV=1

target/riscv: Add additional xlen for address when MPRV=1

As specified in privilege spec:"When MPRV=1, load and store memory
addresses are treated as though the current XLEN were set to MPP’s
XLEN". So the xlen for address may be different from current xlen.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20230614032547.35895-2-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>