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/Lock.h

@@ -48,7 +48,7 @@ public:
         : m_name(name)
     {
     }
-    ~Lock() { }
+    ~Lock() = default;
 
     void lock(Mode = Mode::Exclusive);
 #if LOCK_DEBUG
@@ -144,7 +144,7 @@ private:
 template<typename T>
 class Lockable {
 public:
-    Lockable() { }
+    Lockable() = default;
     Lockable(T&& resource)
         : m_resource(move(resource))
     {