Kernel: Add stubs for missing x86_64 functionality

This adds just enough stubs to make the kernel compile on x86_64. Obviously
it won't do anything useful - in fact it won't even attempt to boot because
Multiboot doesn't support ELF64 binaries - but it gets those compiler errors
out of the way so more progress can be made getting all the missing
functionality in place.

Kernel/VM/ProcessPagingScope.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <Kernel/Arch/x86/InterruptDisabler.h>
+#include <Kernel/Panic.h>
 #include <Kernel/VM/MemoryManager.h>
 #include <Kernel/VM/ProcessPagingScope.h>
 
@@ -20,7 +21,11 @@ ProcessPagingScope::ProcessPagingScope(Process& process)
 ProcessPagingScope::~ProcessPagingScope()
 {
     InterruptDisabler disabler;
+#if ARCH(I386)
     Thread::current()->tss().cr3 = m_previous_cr3;
+#else
+    PANIC("ProcessPagingScope::~ProcessPagingScope() not implemented");
+#endif
     write_cr3(m_previous_cr3);
 }