Feature: Allow to pass flags to verilator when compiling a simulation

[dlobato]

Currently there's no way to pass extra options to verilator, for instance a library path. It would be great to have a option to pass extra flags.

Seems like verilator command is built here and there's no way to pass extra flags:

SpinalHDL/sim/src/main/scala/spinal/sim/VerilatorBackend.scala

Line 333 in 0047f38

val verilatorCmd = s"""${if(isWindows)"verilator_bin.exe" else "verilator"}

[dlobato]

I've implemented extraFlags for verilator:
#140

[Dolu1990]

Thanks for the fix