Add some experimental mainline targets.

These are not really realistic at the moment, but it's nice to get some
targets checked in that can be iterated upon.

- mainline_system: base mk for mainline systemimage. It will evolve to
  inherit other mks than base, and probably get a bunch of things added.
- mainline_system_arm: variant of mainline_system, specifically for
  the 'generic' device.
- mainline_arm: experimental product that also includes the non-system
  parts of aosp. Useful to experiment with the mk split.

Bug: 80410283
Test: lunch mainline_arm64 && m nothing && lunch mainline_system_arm64 && m nothing
Change-Id: I9beaa6f5f9c683733ed76a6d4e5c70b9376a5bb2
Merged-In: I9beaa6f5f9c683733ed76a6d4e5c70b9376a5bb2