Pipe 2 is a DSP pipe that is used to initialize both the DSP hardware (the
application signals to the DSP to initialize) and the application (the DSP
provides the memory location of structures in the shared memory region).

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.