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

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@222702 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed