Add Vivado XSim as a simulator

[andrewb1999]

Adds vivado xsim as a choice of simulator to fud2 because I want to use some encrypted IPs that can't be simulated by Verilator. XSim is slower than verilator by a lot so little reason for others to use it unless they want to use Xilinx floating point IP. It also behaves differently on some verilog constructs, so I had to make some fixes there (these fixes should also help calyx generated verilog work better on all time based (as opposed to cycle based like verilator) simulators).

Not really sure how to update the fud2 test cases but I won't do that until people agree with the other changes anyways.