Add support for symlink_preferred_arch in apex

Some modules rely on symlink_preferred_arch to have expected files
present. This change makes apexs include these symlinks.

Test: m com.android.runtime.debug
      pushd $(mktemp -d)
      mkdir mnt
      unzip $OUT/apex/system/com.android.runtime.debug.apex
      sudo mount -o loop,ro apex_payload.img mnt
      Ensure that mnt/bin/dalvikvm and mnt/bin/dex2oatd both exist and
      are symlinks to mnt/bin/dalvikvm64 and mnt/bin/dex2oatd32
      respectively.

Bug: 119942078
Bug: 122373634
Bug: 123079311
Change-Id: I47868fbedc5bdd3141a836c488f79e91e0a6ddfe