Add new armv8-2a-dotprod arch variant support.

This patch introduces the .mk file required to support build for the new
armv8-2a-dotprod Arm64 arch variant. The file just needs to be present
but does not require to contain anything.

Test: test-art-target on Pixel 4.

Change-Id: I4b6a1e306f9819cf31cd556351bd3e8cf08e2537