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

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

Change-Id: I3f7980a4e236809fa4302fadf6cee65d863c9116
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>