Add special case for finding the in-tree ABI library.

When libcxx is built in-tree with libcxxabi it links against libcxxabi using
the name of the cmake target and not the actual library name. The cmake target
will not work with `find_library()`, so it needs special case handling.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220121 91177308-0d34-0410-b5e6-96231b3b80d8