Kernel: Move Prekernel{CPU, Exceptions}.cpp out of Prekernel directory

This lets us delete the Prekernel.h file, and gets us closer to deleting
the Prekernel from the aarch64 Kernel.

Kernel/Arch/aarch64/CPU.h

@@ -7,3 +7,10 @@
 #pragma once
 
 #define PAGE_MASK (~(FlatPtr)0xfffu)
+
+namespace Prekernel {
+
+void drop_to_exception_level_1();
+void init_prekernel_page_tables();
+
+}