Enable testing for static libc++abi

This change leverages framework changes made in libcxx. See those changes for
more details. (http://reviews.llvm.org/D16544)

Some Mac specific logic for testing against libc++abi had to be moved from
libcxxabi's config.py, as it was overriding choices made in libcxx's config.py.
That logic is now in libcxx's target_info.py.

http://reviews.llvm.org/D16545

llvm-svn: 266729