Adding CONTEXT_BARRIER to force the PPC context to synchronize.

This is just an annotation right now, as it's not actually needed.

src/xenia/cpu/hir/hir_builder.h

@@ -142,6 +142,7 @@ class HIRBuilder {
 
   Value* LoadContext(size_t offset, TypeName type);
   void StoreContext(size_t offset, Value* value);
+  void ContextBarrier();
 
   Value* LoadMmio(cpu::MMIORange* mmio_range, uint32_t address, TypeName type);
   void StoreMmio(cpu::MMIORange* mmio_range, uint32_t address, Value* value);