Add gsi_common.mk

gsi_common.mk is a new makefile file for GSI build targets.
It inherited the content from Mainline by mainline_system.mk.
Also it include other default GSI settings and try to covered
all contents from aosp_base_telephony.mk

It will be used to replace the treble_common*.mk. All
aosp_$arch*.mk will become to inherit from gsi_common.mk.

This patch also contains a new build target gsi_arm64,
which is used to test the gsi_common.mk purely.

Bug: 119747870
Test: compare the out system folder between gsi_arm64 and aosp_arm64
Test: also the gsi_arm64-userdebuge can boot on a Pixel device
Change-Id: I9a013f9407e0fe06b45011e36d5169b94f67148d