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_cpu.h

@@ -12,14 +12,14 @@
 #include "common/common_types.h"
 #include "core/arm/exclusive_monitor.h"
 
-class ARM_Interface;
-
 namespace Kernel {
 class Scheduler;
 }
 
 namespace Core {
 
+class ARM_Interface;
+
 constexpr unsigned NUM_CPU_CORES{4};
 
 class CpuBarrier {