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/CMakeLists.txt

@@ -13,10 +13,11 @@ set(KERNEL_SOURCES
     ACPI/MultiProcessorParser.cpp
     ACPI/Parser.cpp
     AddressSanitizer.cpp
+    Arch/PC/BIOS.cpp
     Arch/i386/CPU.cpp
     Arch/i386/ProcessorInfo.cpp
     Arch/i386/SafeMem.cpp
-    Arch/PC/BIOS.cpp
+    Arch/x86/SmapDisabler.h
     CMOS.cpp
     CommandLine.cpp
     Console.cpp