Check if current ld.config.txt files is backward compatible

The ld.config.txt file is a template to be used for current build as
well as for the VNDK snapshots for old vendor partitions.
To work with the old Android vendor partitions, the ld.config.txt
template must be backward compatible.
This script checks if the current ld.config.txt template modifies or
deletes the vendor directores in every section. If the change is
detected, it will report a build error.

Bug: 112738739
Bug: 111417344
Test: make ld.config.txt
Change-Id: I1211696d6a00a37d531d40c894f1d7e64b210ae5