Updated PACKED to alignas for DexCachePair and TLS data structures am: f15e067185 am: ea4f1046fa

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

Change-Id: Ibe70e4c0e28caa64d67565c3f80cc03b07927343
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>