[libc++] Add UNSUPPORTED markup to atomic test in single-threaded mode

Author: ldionne

libcxx/test/libcxx/atomics/ext-int.verify.cpp

@@ -15,6 +15,7 @@
 // UNSUPPORTED: clang-4, clang-5, clang-6, clang-7, clang-8, clang-9, clang-10
 // UNSUPPORTED: apple-clang-9, apple-clang-10, apple-clang-11, apple-clang-12
 
+// UNSUPPORTED: libcpp-has-no-threads
 // UNSUPPORTED: c++03
 
 #include <atomic>