Use cxxrtl as SoC simulation backend.

[vk2seb]

Working proof-of-concept: #56

I tried cxxrtl in the past as a simulation backend, however it was extremely slow due to a regression in LLVM. Once this regression is commonly 'gone' in most commodity linux distributions, we should consider switching to cxxrtl everywhere. (or, ideally find a shorter term solution and migrate sooner)

Upstream bug: YosysHQ/yosys#4419