Fix MSVC compiler and linker errors

- Fix struct/class forward declaration mismatches (JAIAudience, JASTrack) causing different MSVC mangled names and unresolved symbols
- Add jsystem_stubs.cpp with stubs for JASHeap, JASVoiceBank, J3DShapeTable, JAUSection, JHICommBuf, HIO/HIO2, JOR

include/d/actor/d_a_b_ds.h

@@ -332,8 +332,10 @@ private:
     /* 0x2EDC */ dMsgFlow_c mMsgFlow;
 };
 
+#ifdef __MWERKS__
 cXyz daB_DS_c::getHandPosR() { return mHandPos[1]; }
 cXyz daB_DS_c::getHandPosL() { return mHandPos[0]; }
+#endif
 
 STATIC_ASSERT(sizeof(daB_DS_c) == 0x2F28);