Add --llndk to create_reference_dumps.py for llndk libraries.

Currently, we do abi checks on the system variants of llndk libraries.
Therefore, add an 'llndk' flag to specify that the system variant should be
built.

Bug: 77101345

Test: utils/create_reference_dumps.py --llndk -l libdl produces
      libdl references.

Change-Id: Id4c607674f096daf041520d4a1ca43199d4451da