Adds the BinarySemaphore, CountingSemaphore, and Mutex to the
pw_sync module along with an STL backend for each.
Change-Id: I54a3a64e702202a319ed2c9068bf37412d3fd240
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/24241
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
