ARM: pass MemorySystem separately in the constructor and make System optional

So that unit test can test CPU without constructing the entire system. Also remove hacks in the System class

src/core/core.h

@@ -270,8 +270,6 @@ private:
 
     std::unique_ptr<Service::FS::ArchiveManager> archive_manager;
 
-public: // HACK: this is temporary exposed for tests,
-        // due to WIP kernel refactor causing desync state in memory
     std::unique_ptr<Memory::MemorySystem> memory;
     std::unique_ptr<Kernel::KernelSystem> kernel;
     std::unique_ptr<Timing> timing;