core: Namespace all code in the arm subdirectory under the Core namespace

Gets all of these types and interfaces out of the global namespace.

src/core/core.h

@@ -22,8 +22,6 @@
 #include "video_core/debug_utils/debug_utils.h"
 #include "video_core/gpu.h"
 
-class ARM_Interface;
-
 namespace Core::Frontend {
 class EmuWindow;
 }
@@ -38,6 +36,8 @@ class RendererBase;
 
 namespace Core {
 
+class ARM_Interface;
+
 class System {
 public:
     System(const System&) = delete;