Kernel: Use default con/de-structors

This may seem like a no-op change, however it shrinks down the Kernel by a bit:
.text -432
.unmap_after_init -60
.data -480
.debug_info -673
.debug_aranges 8
.debug_ranges -232
.debug_line -558
.debug_str -308
.debug_frame -40

With '= default', the compiler can do more inlining, hence the savings.
I intentionally omitted some opportunities for '= default', because they
would increase the Kernel size.

Kernel/Time/HardwareTimer.h

@@ -47,7 +47,7 @@ class HardwareTimer;
 class HardwareTimerBase
     : public RefCounted<HardwareTimerBase> {
 public:
-    virtual ~HardwareTimerBase() { }
+    virtual ~HardwareTimerBase() = default;
 
     // We need to create a virtual will_be_destroyed here because we derive
     // from RefCounted<HardwareTimerBase> here, which means that RefCounted<>