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.cpp

@@ -25,6 +25,8 @@ namespace Core {
 
 /*static*/ System System::s_instance;
 
+System::System() = default;
+
 System::~System() = default;
 
 /// Runs a CPU core while the system is powered on

src/core/core.h

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