Support Verilator 5

[RickLuiken]

Following from the discussion in the fork from KU Leuven. I would like to have support for Verilator 5. Verilator 5 has better performance, support for timing constructs and support for assertions. It would also allow projects using the snitch cluster to use verilator 5. I think in general it would be useful to support more recent versions of tools, as this makes the upgrading process later down the line a lot easier (make small steps instead of big ones).

Upgrading to verilator 5 would involve the following steps:

• Add the -no-timing flag to the current tests, which disables the new timing features in verilator 5
• Add verilated_threads.o to the verilator build targets OR compile both the systemverilog and cpp sources using verilator, so verilator can manage which verilator files should be included

If you have any questions or comments, please let me know!

[colluca]

Dear @RickLuiken,

If you could provide a PR for this feature targeting Verilator 5.006, it would be well accepted and we would be willing to review and merge it.

Unfortunately we do not have enough resources to implement this feature ourselves at the moment.

[RickLuiken]

Great, I will provide the PR after the christmas break!