pw_sync_baremetal: Add simple mutex implementation

Adds a dummy mutex implementation for baremetal configurations.

The provided implementation makes a single attempt to acquire the
lock and asserts if it is unavailable. It does not perform interrupt
masking or disable global interrupts, so this implementation does
not support simultaneous multi-threaded environments including IRQs,
and is only meant to prevent data corruption.

Change-Id: Id122f890a37822c3b752b810e0e687f19f8c9dbd
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/45721
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Prashanth Swaminathan <prashanthsw@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>