Modules didn't correctly define their dependencies before, which relied
on the frontends implicitly including every module for linking to
succeed.
Also changed every target_link_libraries call to specify visibility of
dependencies to avoid leaking definitions to dependents when not
necessary.
This commit:
* Adds a new subproject, audio_core.
* Defines structures that exist in DSP shared memory.
* Hooks up various other parts of the emulator into audio core.
This sets the foundation for a later HLE DSP implementation.
bunnei