am: Implement UserChannel parameters

Used by the Super Mairo 3D All-Stars collection.

src/core/core.cpp

@@ -562,6 +562,8 @@ struct System::Impl {
 
     std::array<Core::GPUDirtyMemoryManager, Core::Hardware::NUM_CPU_CORES>
         gpu_dirty_memory_write_manager{};
+
+    std::deque<std::vector<u8>> user_channel;
 };
 
 System::System() : impl{std::make_unique<Impl>(*this)} {}
@@ -1036,6 +1038,10 @@ void System::ExecuteProgram(std::size_t program_index) {
     }
 }
 
+std::deque<std::vector<u8>>& System::GetUserChannel() {
+    return impl->user_channel;
+}
+
 void System::RegisterExitCallback(ExitCallback&& callback) {
     impl->exit_callback = std::move(callback);
 }