pw_rpc: Add synchronized channel output

This adds an channel output wrapper which synchronizes a ChannelOutput's
operations with a sync::Mutex, and updates the host target's system RPC server
to use it.

Change-Id: I3d2407a8c81a8f0370fd714ded850293d7030209
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/28540
Commit-Queue: Alexei Frolov <frolv@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_chrono_stl/CMakeLists.txt b/pw_chrono_stl/CMakeLists.txt
new file mode 100644
index 0000000..f0b6197
--- /dev/null
+++ b/pw_chrono_stl/CMakeLists.txt
@@ -0,0 +1,20 @@
+# Copyright 2020 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+include($ENV{PW_ROOT}/pw_build/pigweed.cmake)
+
+pw_add_module_library(pw_chrono_stl.system_clock
+  IMPLEMENTS_FACADES
+    pw_chrono.system_clock
+)