target/riscv: Extend pc for runtime pc write

target/riscv: Extend pc for runtime pc write

In some cases, we must restore the guest PC to the address of the start of
the TB, such as when the instruction counter hits zero. So extend pc register
according to current xlen for these cases.

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: 20220120122050.41546-8-zhiwei_liu@c-sky.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>