Fix linking with just-built libc++abi (added in r214037).

Pass target name ("cxxabi") to target_link_libraries(cxx ...) to ensure
that linker is able to locate just-built libc++abi in the build tree,
instead of relying on "-lc++abi" linker flag.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@214114 91177308-0d34-0410-b5e6-96231b3b80d8