Use dex2oat64 on 64-bit-only builds

dexopt defaults to dex2oat32 unless instructed otherwise. Since
dex2oat32 doesn't exist on a 64-bit-only build, make sure dex2oat64
is selected instead.

Test: boot aosp_cf_x86_64_only_phone and check that installd doesn't
  try to use dex2oat32
Change-Id: Ia67e746894684a52a4e5b765bfde0f6dd0efbf6e