[libcxxabi] Refactor building and testing libc++abi without threads

Summary:
This patch adds CMake support for building and testing libc++abi without threads.


1. Add `LIBCXXABI_ENABLE_THREADS` option to CMake.
2. Propagate `LIBCXXABI_ENABLE_THREADS` to lit via lit.site.cfg.in
3. 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.



Reviewers: danalbert, mclow.lists, jroelofs

Reviewed By: jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6393