Soong: Add synopsis to several modules under the cc package.

Added synopsis to the following modules under cc package:
    * llndk_headers
    * llndk_library
    * vendor_public_library
    * toolchain_library

Bug: b/128337482
Test: Generated the documentation and verified that the synopsis was
added to each of the module.

Change-Id: I0f6e3b01b478d46afeacbe6fc8e05f96d6217c7d