[ORC-RT][ORC][MachO] Refactor dlsym to extract a reusable bulk-lookup API.

MachOPlatformRuntimeState::lookupSymbols encapsulates the approach used in
dlsym in bb41fc68, but generalizes it to multiple symbols:
  1. try to resolve symbols locally
  2. issue a push-request for any unresolved symbols
  3. re-try local resolution

In the future lookupSymbols can serve as the basis for a public bulk lookup
API in the ORC runtime.