Ensure that all model symbols from static libraries would be linked in and no symbol garbage collection occurs

[rhornig]

Platform

• OMNeT++ version: 6.0.3
• OS: all platforms

Describe the bug
Like with #1055, --whole-archive can cause a lot of issues. It would be great to get rid of the --whole-archive workaround and instead force all registered symbols (like modules and NED functions) to be marked as 'referenced' to avoid automatic symbol garbage collection when linked into an executable. (Linking into a shared lib does not cause an issue as all exported symbols are automatically deemed 'referenced' in case of the creation of a shared lib). The same is not true if an executable is created. In that case the linker adds only symbols that it thinks was referenced from the main executable.

Possible solution
retain and used attributes might be enough to force the linker to always avoid garbage collecting these symbols. If that works, we can get rid of the --whole-archive menchanism.