Kernel: Enable PAE (Physical Address Extension)

Introduce one more (CPU) indirection layer in the paging code: the page
directory pointer table (PDPT). Each PageDirectory now has 4 separate
PageDirectoryEntry arrays, governing 1 GB of VM each.

A really neat side-effect of this is that we can now share the physical
page containing the >=3GB kernel-only address space metadata between
all processes, instead of lazily cloning it on page faults.

This will give us access to the NX (No eXecute) bit, allowing us to
prevent execution of memory that's not supposed to be executed.

Kernel/Thread.cpp

@@ -100,7 +100,6 @@ Thread::Thread(Process& process)
         m_tss.esp0 = m_kernel_stack_top;
         kprintf("Allocated ring0 stack @ %p - %p\n", m_kernel_stack_base, m_kernel_stack_top);
     }
-    m_process.page_directory().update_kernel_mappings();
 
     // HACK: Ring2 SS in the TSS is the current PID.
     m_tss.ss2 = m_process.pid();