SerialWidget's target-time behavior influenced by simulation stalls

[novobruna]

The SerialWidget inBuf starts to be filled in a modified design (modified to also stall when myStall is high) while the stall signal is active. As a result, the inBuf already contains an element once the simulation is restored and this is immediately sent to the target. Instead, in the golden design, the inBuf sends the element 2 rocket-chip cycles later than in the modified design because the inBuf had to be filled first