target/riscv: Don't save pc when exception return

target/riscv: Don't save pc when exception return

As pc will be written by the xepc in exception return, just ignore
pc in translation.

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: 20220120122050.41546-3-zhiwei_liu@c-sky.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>