sal/arm: compiler warning fixed (see -Wdelete-non-virtual-dtor)

This commit introduces a virtual dtor in the ArmCPUState class to
prevent the compiler from producing the following warning:
 "deleting object of polymorphic class type 'fail::Gem5ArmCPU' which
  has non-virtual destructor might cause undefined behaviour"

Change-Id: I5029662064e72ae87a9f0e36aeaa309e4cb2291b

src/core/sal/arm/ArmCPUState.hpp

@@ -18,6 +18,7 @@ public:
 	 * @return the current lr
 	 */
 	virtual address_t getLinkRegister() const = 0;
+	virtual ~ArmCPUState() { }
 };
 
 // TODO: Enum for misc registers