FreeDV: Disable LPCNet on 32-bit ARM.

[tmiw]

This PR disables LPCNet for 32-bit builds to allow FreeDV to build on 32-bit Bookworm. 64-bit builds should still include it, though (if I'm understanding Ansible correctly, anyway; I'm having difficulty testing this change my own Pi due to it complaining about not being able to find the latest version of CMake on GitHub).

For context: FreeDV traditionally required LPCNet to build/execute even if the machine was too slow to be able to use the FreeDV 2020 modes. As 2020/2020B modes may be deprecated soon, it was decided to perform only minimal maintenance on the FreeDV version of the LPCNet library (meaning that changes to allow it to build on the 32-bit ARM version of Bookworm are probably not coming any time soon) and uncouple the main FreeDV application from LPCNet.