Use PlusArgArtefacts.append to avoid emulator.cc hacks in next RC bump

[jwright6323]

With this PR (chipsalliance/rocket-chip#2453) we can just add string plusargs or plusargs with no default by calling PlusArgArtefacts.append. This should go in SimUART (to add the uartlog plusarg) and SimSPIFlashModel (there's a comment we can delete explaining what needs to be changed). Additionally you could have multiple logs for multiple UARTs by passing a string parameter to the SimUART verilog module, instead of hardcoding it to uartlog.