Add build target aosp_arm_64b_a

The GSI build target is for legacy devices which upgrded from
O/O-MR1. The configure is:

- ARM 32bits user space
- 64bits binder interface
- non-system-as-root partition layout

Bug: 117576577
Test: lunch aosp_arm_64b_a-userdebug; make -j
Change-Id: Ia51e154863cbf0a103d7d4fc96d08db883625568