arduino: Add baremetal sync backend
arduino: Add baremetal sync backend

Resolve build breakage by installing baremetal backend for spin lock.

Change-Id: I1bc4caa899266677413fd51213e80557382af33e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/31501
Pigweed-Auto-Submit: Prashanth Swaminathan <7074f112da8ec89069105f3b18a4ce46a4722488@google.com>
Reviewed-by: Anthony DiGirolamo <4cc40125edc24ec5bc72df7917ea0b403ee8cebe@google.com>
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed