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

Summary:
This patch adds support for building/testing libc++ with an ABI library that the linker would not normally find.

• CMAKE_LIBRARY_PATH is used to specify the list of search directories.
• The ABI library is now found using find_library instead of assuming its along the linker's search path.
• CMAKE_LIBRARY_PATH is passed to our LIT config as library_paths.
• For each path in library_paths the following flags are added -L<path> -Wl,-rpath -Wl,<path>

Some changes in existing behavior were also added:

• target_link_libraries is now passed the ABI library file instead of the library name. Ex target_link_libraries(cxx "/usr/lib/libc++abi.so") vs target_link_libraries(cxx "c++abi").
• -Wl,-rpath -Wl,<path> is now used on OSX to link to libc++ instead of env['DYLD_LIBRARY_PATH'] if use_system_lib=False.

Reviewers: mclow.lists, danalbert, EricWF

Reviewed By: EricWF

Subscribers: emaste, cfe-commits

Differential Revision: http://reviews.llvm.org/D5038