loader: Can't dlclose until all done with the layers

The loader was freeing the layers (dlclose) at the
bottom of the DestroyInstance chain. Would get a
segfault when returning from DestroyInstance_Bottom
as layers would try to run that are no longer in memory.

Change-Id: I25cc611fed9d8b3a90fd2652e53cd450daefa08c
(cherry picked from commit 3160835e)