Updated PACKED to alignas for DexCachePair and TLS data structures am: f15e067185

Original change: https://android-review.googlesource.com/c/platform/art/+/2962593

Change-Id: I0d5635bbd629718a9875a162013b74d8815ffb8d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>