Add support for using SymbiFlow open source tooling instead of Vivado

[mithro]

It would be awesome if you could support using the SymbiFlow open source FPGA toolchain as an alternative to Vivado.

As the SymbiFlow project would be interested in have some anasymod designs as benchmarks in the fpga-tool-perf and potentially even in the examples repository we would be willing to help figure out incompatibilities between the output of anasymod and the open source tooling.

[jopdorp]

Where does it currently fail? I'm looking at compatibility for iverilog and quartus.

For us it fails at the $log2 reimplementation