This adds a function to the RTS linker API which lets the
user check the status of dynamically linked objects.

It was initially proposed by @afarmer in D2068.
It's useful for testing the linker and also for detecting retention
problems in production.

It takes a path, because it's easier to use path as key instead of producing
some stable handle.

It returns an enum instead of bool, because I see no reason for destroying
information. All the complexity is already out in the open, so there's
nothing to save the users from.

Test Plan: ./validate

Reviewers: simonmar, Phyx, bgamari, austin, erikd

Reviewed By: Phyx, bgamari

Subscribers: rwbarton, afarmer, thomie

Differential Revision: https://phabricator.haskell.org/D3963