Kernel: Move x86-specific init sequence code to the x86/Arch directory

The code in init.cpp is specific to the x86 initialization sequence, so
move it to the Arch/x86 directory in the same fashion like the aarch64
pattern.

Kernel/CMakeLists.txt

@@ -309,13 +309,14 @@ set(KERNEL_SOURCES
     UserOrKernelBuffer.cpp
     WaitQueue.cpp
     WorkQueue.cpp
-    init.cpp
     kprintf.cpp
 )
 
 if ("${SERENITY_ARCH}" STREQUAL "i686" OR "${SERENITY_ARCH}" STREQUAL "x86_64")
     set(KERNEL_SOURCES
         ${KERNEL_SOURCES}
+        Arch/x86/init.cpp
+
         Arch/Processor.cpp
 
         Arch/x86/common/Interrupts/APIC.cpp