s/LIBCXXABI_SINGLE_THREADED/LIBCXXABI_HAS_NO_THREADS/ for consistency with libcxx

Also remove the audotedection part so that if you're crazy enough to want a
single-threaded abi library, you'll say so explicitly in the build.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@217262 91177308-0d34-0410-b5e6-96231b3b80d8