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>
