[libcxxabi] Refactor test timing logic and disable by default.

Summary:
When using LIT the timing output is entirely unused but introduces a dependency on `<chrono>`. When libc++ is built without a montonic clock this causes some of the tests to fail.
This patch factors out all of the timing logic into `support/timer.hpp` and disables it by default. To enable the timing you must define `LIBCXXABI_TIME_TESTS`.



Reviewers: mclow.lists, danalbert, jroelofs

Reviewed By: jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6391

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@222701 91177308-0d34-0410-b5e6-96231b3b80d8
[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

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