pw_sync_baremetal: Add scaffolding for baremetal

Adds a dummy spin lock backend implementation for baremetal
configuration.

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: I4d44b2dd5cc6f38be1188a9f480f61c6d042b855
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/30520
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Prashanth Swaminathan <prashanthsw@google.com>
diff --git a/pw_sync_baremetal/public_overrides/pw_sync_backend/spin_lock_inline.h b/pw_sync_baremetal/public_overrides/pw_sync_backend/spin_lock_inline.h
new file mode 100644
index 0000000..65a64b0
--- /dev/null
+++ b/pw_sync_baremetal/public_overrides/pw_sync_backend/spin_lock_inline.h
@@ -0,0 +1,16 @@
+// Copyright 2021 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.
+#pragma once
+
+#include "pw_sync_baremetal/spin_lock_inline.h"