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/CMakeLists.txt

@@ -222,7 +222,6 @@ set(KERNEL_SOURCES
     Memory/AnonymousVMObject.cpp
     Memory/InodeVMObject.cpp
     Memory/MemoryManager.cpp
-    Memory/PageDirectory.cpp
     Memory/PhysicalPage.cpp
     Memory/PhysicalRegion.cpp
     Memory/PhysicalZone.cpp