QEMU: atomic test sometimes fail when --singlestep

on steps/13, run qemu as:
`qemu-system-riscv32 -nographic -machine virt -kernel m.elf --singlestep` 
sometimes fails with `++++++++82: "ret" != "1"`

```
$ qemu-system-riscv32 --version
QEMU emulator version 4.1.50 (v4.1.0-838-ge43dc48e6e-dirty)
Copyright (c) 2003-2019 Fabrice Bellard and the QEMU Project developers
```