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

rust/config/arm64_device.go

@@ -33,6 +33,7 @@ var (
 	Arm64ArchVariantRustFlags = map[string][]string{
 		"armv8-a":  []string{},
 		"armv8-2a": []string{},
+		"armv8-2a-dotprod": []string{},
 	}
 )