loader: fix d-tor call order

In the case when there are multiple dependencies on
the same library in the local_group the unload may
in some situations (covered now by tests) result
calling d-tors for some libraries prematurely.

In order to have correct call order loader checks if this
is last dependency in local group before adding it to BFS
queue.

Bug: http://b/35201832
Test: bionic-unit-tests --gtest_filter=dl*:Dl*
Test: bionic-unit-tests-glibc --gtest_filter=dl*
Change-Id: I4c6955b9032acc7147a51d9f09b61d9e0818700c