target/riscv: Calculate address according to XLEN

target/riscv: Calculate address according to XLEN

Define one common function to compute a canonical address from a register
plus offset. Merge gen_pm_adjust_address into this function.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220120122050.41546-14-zhiwei_liu@c-sky.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>