pw_libcxx: Clang support for Arm Cortex-M33

Bug: b/358108912
No-Tree-Checks: true
Change-Id: Ib37a720febb75c3778307eb59a99b30f07670e7c
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/228391
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Original-Reviewed-by: Ted Pudlik <tpudlik@google.com>
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/llvm-integration/+/235150
Commit-Queue: Prabhu Karthikeyan Rajasekaran <prabhukr@google.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>
diff --git a/pw_libcxx/BUILD.bazel b/pw_libcxx/BUILD.bazel
index c9dbbfc..8254216 100644
--- a/pw_libcxx/BUILD.bazel
+++ b/pw_libcxx/BUILD.bazel
@@ -12,6 +12,8 @@
 # License for the specific language governing permissions and limitations under
 # the License.
 
+package(default_visibility = ["//visibility:public"])
+
 cc_library(
     name = "pw_libcxx",
     srcs = [
@@ -26,3 +28,10 @@
         "//conditions:default": ["@platforms//:incompatible"],
     }),
 )
+
+filegroup(
+    name = "cxx_lib_configs",
+    srcs = [
+        "include/__external_threading",
+    ],
+)