core: Make System's default constructor private

This makes it a compilation error to construct additional instances of
the System class directly, preventing accidental wasteful constructions
over and over.

src/core/core.h

@@ -168,6 +168,8 @@ public:
     }
 
 private:
+    System();
+
     /// Returns the currently running CPU core
     Cpu& CurrentCpuCore();