lineage: targets: Add support for Legacy GSIs

* This is needed for VNDK27, and other older,
  hackier Treble implementations.

* We can't include `legacy_gsi_release.mk` as it's included
  in the `aosp_${ARCH}_ab` targets themselves, unlike the standard
  `aosp_${ARCH}` targets which guard `gsi_release.mk`'s inclusion
   with `aosp_${ARCH}` product guards.

Change-Id: I16c5983c18110b24eef84e382e0ae6852efb03c3