This patch adds CMake support for building and testing libc++abi without threads.
- Add LIBCXXABI_ENABLE_THREADS option to CMake.
- Propagate LIBCXXABI_ENABLE_THREADS to lit via lit.site.cfg.in
- Configure tests for `LIBCXXABI_ENABLE_THREADS=OFF
Currently the test suite does not work when libc++abi is built without threads because that information does not propagate to the test suite.