Not truly a new package, but rather a new backend next to the existing
`mcode` and `llvm` ones. Adding the GCC backend too, as it is the only
one that supports code coverage report generation. For more info see:
https://devsaurus.github.io/ghdl_gcov/ghdl_gcov.html.
Co-authored-by: Rowan Goemans <goemansrowan@gmail.com>