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.

* AudioCore: Implement time stretcher

* fixup! AudioCore: Implement time stretcher

* fixup! fixup! AudioCore: Implement time stretcher

* fixup! fixup! fixup! AudioCore: Implement time stretcher

* fixup! fixup! fixup! fixup! AudioCore: Implement time stretcher

* fixup! fixup! fixup! fixup! fixup! AudioCore: Implement time stretcher

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.