Having the libc6-dbg package installed in my Ubuntu 18.04 system, I still don't see any function names inside libc unless I specify the path to debug symbols explicitly with --debugPaths /usr/lib/debug. However, this path is somewhat standard, and discussion under #52 implies it's already in the default search path, which doesn't seem to be the case. I'm using the 1.1.0 AppImage distribution.

UPD: it looks like the issue is only present when using AppImage; anyway, the binary built from master doesn't need explicit paths.