Add support for cxxrtl as a simulation backend

[dkozel]

Currently Verilator is the only supported simulation engine. It is fast and Free and Open Source Software and remains an excellent default option. Recently cxxrtl was implemented and merged into Yosys and provides an alternative backend. Currently it runs slower than Verilator but is under very active development and has a rapidly expanding feature set and ecosystem which make it desirable to support.

An example of using cxxrtl is included in this repository: https://github.com/tomverbeure/cxxrtl_eval

[dkozel]

@miek has made an example implementation that probably has some useful pieces or inspiration points. It is directly driving an IQ bus rather than AXI/Wishbone, but that should be a fairly simple piece to adapt.
https://github.com/miek/gr-cxxrtl-experiment