embOS backend modules

Pigweed provides native embOS backends for several of its core OS abstraction layers.

OS backend modules

API group	Backend module	Description
Time	pw_chrono_embos	Non-overflowing 64-bit SystemClock implementation.
Synchronization	pw_sync_embos	Type-safe Mutex, TimedMutex, InterruptSpinLock, Semaphores, and ThreadNotification.
Threading	pw_thread_embos	Thread creation, stack allocation, sleep, yield, iteration, and snapshot utilities.