coverage support

[LeFl0w]

hi, it is more like an information for you than a real pull request.

Thanks to GHDL with GCC backend, we can compute coverage.
You can see in the diff that there is not many changes , the only 'difficulty' is to recompile GHDL with gcc backend.
Feel free to merge it (be careful as if your ghdl has not got covergage builtin, the simulation won't work) or to refuse it and get inspired to include coverage in your repository later on if wanted.

You can have a look at the lcov coverage output: html.zip

cheers.