core: Don't construct instance of Core::System, just to access its live instance

This would result in a lot of allocations and related object
construction, just to toss it all away immediately after the call.

These are definitely not intentional, and it was intended that all of
these should have been accessing the static function GetInstance()
through the name itself, not constructed instances.

src/core/core.cpp

@@ -29,7 +29,7 @@ System::~System() = default;
 
 /// Runs a CPU core while the system is powered on
 static void RunCpuCore(std::shared_ptr<Cpu> cpu_state) {
-    while (Core::System().GetInstance().IsPoweredOn()) {
+    while (Core::System::GetInstance().IsPoweredOn()) {
         cpu_state->RunLoop(true);
     }
 }