[cmake] Add a config option LIBCXX_HAS_WIN32_THREAD_API for enforcing win32 threads

This allows keeping libcxx using win32 threads even if a
version of pthread.h is installed.

This matches the existing cmake option LIBCXX_HAS_PTHREAD_API.

Also add missing documentation about the internal define
_LIBCPP_HAS_THREAD_API_WIN32.

Differential Revision: https://reviews.llvm.org/D41764

llvm-svn: 321896