Kernel: Add .unmap_after_init section for code we don't need after init

You can now declare functions with UNMAP_AFTER_INIT and they'll get
segregated into a separate kernel section that gets completely
unmapped at the end of initialization.

This can be used for anything we don't need to call once we've booted
into userspace.

There are two nice things about this mechanism:

- It allows us to free up entire pages of memory for other use.
  (Note that this patch does not actually make use of the freed
  pages yet, but in the future we totally could!)

- It allows us to get rid of obviously dangerous gadgets like
  write-to-CR0 and write-to-CR4 which are very useful for an attacker
  trying to disable SMAP/SMEP/etc.

I've also made sure to include a helpful panic message in case you
hit a kernel crash because of this protection. :^)

Kernel/VM/MemoryManager.h

@@ -142,6 +142,7 @@ public:
     PageFaultResponse handle_page_fault(const PageFault&);
 
     void protect_readonly_after_init_memory();
+    void unmap_memory_after_init();
 
     static void enter_process_paging_scope(Process&);
     static void enter_space(Space&);