Kernel: Move SMAP disabler RAII helper to its own file

Added this in a new directory called Kernel/Arch/x86/ where stuff
that applies to both i386 and x86_64 can live.

Kernel/Thread.cpp

@@ -29,6 +29,7 @@
 #include <AK/StringBuilder.h>
 #include <AK/Time.h>
 #include <Kernel/Arch/i386/CPU.h>
+#include <Kernel/Arch/x86/SmapDisabler.h>
 #include <Kernel/Debug.h>
 #include <Kernel/FileSystem/FileDescription.h>
 #include <Kernel/KSyms.h>