riscv: Set xPIE to 1 after xRET

riscv: Set xPIE to 1 after xRET

When executing an xRET instruction, supposing xPP holds the
value y, xIE is set to xPIE; the privilege mode is changed to y;
xPIE is set to 1. But QEMU sets xPIE to 0 incorrectly.

Signed-off-by: Yiting Wang <yiting.wang@windriver.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>