[vm/ffi] Add `dladdr` embedder callback

> When the dart profiler walks stacks and hits addresses, it wants to do reverse-lookup of address to symbols. It already uses `dladdr` in that case, but if the code wasn't loaded via `dlopen()` but other mechanisms, then it can make sense to call out to the embedder for that. Though this isn't urgent to do.

Posted by @mkustermann in https://dart-review.googlesource.com/c/sdk/+/361881.