Introduce new Arm64 arch variant with dot product features

In order to rely on `-march`/`-mcpu` compiler flags for feature
detection, we introduce a new arch variant based on armv8.2 with the
addition of dot product features.

Test: test-art-target on Pixel 4.

Change-Id: I4d97db6129e2cd718a2b21008d36ec767739f925