[libcxxabi] option for c11 threads api

[nickdesaulniers]

libcxxabi has the cmake options LIBCXXABI_ENABLE_THREADS and LIBCXXABI_HAS_PTHREAD_API.

I think it would be nice if there was an option for using c11 threads. In the case of llvm-libc, pthreads is a much larger surface area than pthreads. We have c11 threads support, but not yet all of pthreads.

Perhaps a cmake option for libcxxabi and code changes to use c11 threads instead of pthreads would help us boostrap a c++ runtime on top of llvm-libc.

cc @petrhosek