[libcxx] Use __config_site to configure the test suite features.

Summary:
This patch changes the tests to use the "__config_site" header if present instead of manually configuring for each option. This patch also removes the test flags for configuring some of these options. For example "lit -sv --param=enable_threads=OFF" no longer works. However lit will still correctly configure if  the CMake option "-DLIBCXX_ENABLE_THREADS=OFF" is given at build time. 

This patch will fix the libc++abi test configuration for `LIBCXX_ABI_VERSION` and `LIBCXX_ABI_UNSTABLE` one we teach it about 'project_obj_dir' . I would like to land this ASAP to prevent more work blockage.

Reviewers: mclow.lists, danalbert, eugenis, ed, jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13714

llvm-svn: 250308