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

[Eric Fiselier]

Hi mclow.lists, danalbert, cbergstrom,

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.

http://reviews.llvm.org/D5860

Files:
  cmake/Modules/HandleLibCXXABI.cmake
  lib/CMakeLists.txt
  test/lit.cfg
  test/lit.site.cfg.in
  www/index.html
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