r192075 broke the buildbot at

    http://lab.llvm.org:8013/builders/libcxx_clang-x86_64-darwin11-RA

lit.py: <string>:230: note: inferred use_system_lib as: False
lit.py: <string>:247: fatal: C++ ABI setting None unsupported for tests

cxx_abi is geting set to None, and the lit script errors out shortly after
that.  This patch changes the default of cxx_abi from None to 'libcxxabi'.
This is likely not the right way to fix this problem.  However it gets the
buildbot running again.  Improvements to this fix are welcome.

llvm-svn: 192609