Kernel: Add mechanism to make some memory read-only after init finishes

You can now use the READONLY_AFTER_INIT macro when declaring a variable
and we will put it in a special ".ro_after_init" section in the kernel.

Data in that section remains writable during the boot and init process,
and is then marked read-only just before launching the SystemServer.

This is based on an idea from the Linux kernel. :^)

Kernel/Arch/i386/CPU.h

@@ -34,6 +34,8 @@
 #include <Kernel/VirtualAddress.h>
 #include <LibC/sys/arch/i386/regs.h>
 
+#define READONLY_AFTER_INIT __attribute__((section(".ro_after_init")))
+
 #define PAGE_SIZE 4096
 #define GENERIC_INTERRUPT_HANDLERS_COUNT (256 - IRQ_VECTOR_BASE)
 #define PAGE_MASK ((FlatPtr)0xfffff000u)