[gn] port 7162fd7

Author: nico

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -761,10 +761,14 @@ if (current_toolchain == default_toolchain) {
       "__thread/id.h",
       "__thread/jthread.h",
       "__thread/poll_with_backoff.h",
+      "__thread/support.h",
+      "__thread/support/c11.h",
+      "__thread/support/external.h",
+      "__thread/support/pthread.h",
+      "__thread/support/windows.h",
       "__thread/this_thread.h",
       "__thread/thread.h",
       "__thread/timed_backoff_policy.h",
-      "__threading_support",
       "__tree",
       "__tuple/make_tuple_types.h",
       "__tuple/pair_like.h",