On supported platforms allow calling dlvsym

[adityashah1212]

dlvsym allows to load specific versions of the symbol. This a GNU thing though, so all platforms may not support it, but I believe it should be possible support on platforms that do.

[nagisa]

Sounds similar to #36. Since we nowadays have OS-specific os::unix::Library, I guess we could explore adding extension traits that have these methods to under os somewhere… Unlike with dlmopen where converting to/from a native handle makes some sense, can't really do that sensibly for dlvsym… still would have to use the OS-specific extension trait in order to use the method though.