[libcxx] Redo adding support for building and testing with an ABI library not along linker paths

Summary:
This is the second attempt at allowing for the use of libraries that the linker cannot find. The first attempt used `CMAKE_LIBRARY_PATH` and `find_library` to select which ABI library should be used. There were a number of problems with this approach:

- `find_library` didn't work with cmake targets (ie in-tree libcxxabi build)
- It wasn't always possible to determine where `find_library` actually found your library.
- `target_link_libraries` inserted the path of the ABI library into libc++'s RPATH when `find_library` was used.
- Linking libc++ and it's ABI library is a special case. It's a lot easier to keep it simple. 

After discussion with @cbergstrum a new approach was decided upon.
This patch achieve the same ends by simply using `LIBCXX_CXX_ABI_LIBRARY_PATH` to specify where to find the library (if the linker won't find it). When this variable is defined it is simply added as a library search path when linking libc++. It is a lot easier to duplicate this behavior in LIT. It also prevents libc++ from being linked with an RPATH.






Reviewers: mclow.lists, cbergstrom, chandlerc, danalbert

Reviewed By: chandlerc, danalbert

Subscribers: chandlerc, cfe-commits

Differential Revision: http://reviews.llvm.org/D5860

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220157 91177308-0d34-0410-b5e6-96231b3b80d8