Attempt to override broken buildbot config for libc++abi.
The buildbots were changed to pass -DLIBCXX_CXX_ABI=libcxxabi, but
they don't provide an include path for the library, so cxxabi.h is
never found while building libc++.
This is a temporary change until the buildbots are updated or until
D63883 lands in a form that unbreaks the bots