Kernel: Move Memory/PageDirectory.{cpp,h} to arch-specific directory

The handling of page tables is very architecture specific, so belongs
in the Arch directory. Some parts were already architecture-specific,
however this commit moves the rest of the PageDirectory class into the
Arch directory.

While we're here the aarch64/PageDirectory.{h,cpp} files are updated to
be aarch64 specific, by renaming some members and removing x86_64
specific code.

Kernel/Arch/aarch64/CPU.h

@@ -12,9 +12,14 @@
 namespace Kernel {
 
 void initialize_exceptions();
-void init_page_tables();
-void unmap_identity_map();
 void panic_without_mmu(StringView);
 void dbgln_without_mmu(StringView);
 
+namespace Memory {
+
+void init_page_tables();
+void unmap_identity_map();
+
+}
+
 }