Kernel: Start implementing purgeable memory support

It's now possible to get purgeable memory by using mmap(MAP_PURGEABLE).
Purgeable memory has a "volatile" flag that can be set using madvise():

- madvise(..., MADV_SET_VOLATILE)
- madvise(..., MADV_SET_NONVOLATILE)

When in the "volatile" state, the kernel may take away the underlying
physical memory pages at any time, without notifying the owner.
This gives you a guilt discount when caching very large things. :^)

Setting a purgeable region to non-volatile will return whether or not
the memory has been taken away by the kernel while being volatile.
Basically, if madvise(..., MADV_SET_NONVOLATILE) returns 1, that means
the memory was purged while volatile, and whatever was in that piece
of memory needs to be reconstructed before use.

Kernel/VM/AnonymousVMObject.h

@@ -3,7 +3,7 @@
 #include <Kernel/VM/PhysicalAddress.h>
 #include <Kernel/VM/VMObject.h>
 
-class AnonymousVMObject final : public VMObject {
+class AnonymousVMObject : public VMObject {
 public:
     virtual ~AnonymousVMObject() override;
 
@@ -11,9 +11,11 @@ public:
     static NonnullRefPtr<AnonymousVMObject> create_for_physical_range(PhysicalAddress, size_t);
     virtual NonnullRefPtr<VMObject> clone() override;
 
-private:
+protected:
     explicit AnonymousVMObject(size_t);
     explicit AnonymousVMObject(const AnonymousVMObject&);
+
+private:
     AnonymousVMObject(PhysicalAddress, size_t);
 
     AnonymousVMObject& operator=(const AnonymousVMObject&) = delete;