Move UDSPControl structure into DSP.h .
Move mixer construction to AudioCommon.cpp.
Delete AudioCommon::UseJIT().
Move audio handling out of DSP emulation.

This is good for a couple of reasons: one, it gets rid of duplicated code,
and two, DSP emulation shouldn't need to interact with audio in the first
place.
Merge pull request #220 from magumagu/audio-handling-cleanup-v2

Audio handling cleanup v2