Kernel: Use a shared physical page for zero-filled pages until written

This patch adds a globally shared zero-filled PhysicalPage that will
be mapped into every slot of every zero-filled AnonymousVMObject until
that page is written to, achieving CoW-like zero-filled pages.

Initial testing show that this doesn't actually achieve any sharing yet
but it seems like a good design regardless, since it may reduce the
number of page faults taken by programs.

If you look at the refcount of MM.shared_zero_page() it will have quite
a high refcount, but that's just because everything maps it everywhere.
If you want to see the "real" refcount, you can build with the
MAP_SHARED_ZERO_PAGE_LAZILY flag, and we'll defer mapping of the shared
zero page until the first NP read fault.

I've left this behavior behind a flag for future testing of this code.

Kernel/VM/PhysicalPage.h

@@ -60,6 +60,8 @@ public:
 
     u16 ref_count() const { return m_retain_count; }
 
+    bool is_shared_zero_page() const;
+
 private:
     PhysicalPage(PhysicalAddress paddr, bool supervisor, bool may_return_to_freelist = true);
     ~PhysicalPage() {}