Replace uses of annotation `[[gnu::unused]]` with `[[maybe_unused]]`. am: 7b5829d07f am: 9dd6e8004e am: 4c23376838

Original change: https://android-review.googlesource.com/c/platform/art/+/2458350

Change-Id: I54304255a602914ffde416f438e132c31f7c8b53
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>