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/init.cpp

@@ -296,6 +296,9 @@ void init_stage2(void*)
 
     load_kernel_symbol_table();
 
+    // NOTE: Everything marked READONLY_AFTER_INIT becomes non-writable after this point.
+    MM.protect_readonly_after_init_memory();
+
     int error;
 
     // FIXME: It would be nicer to set the mode from userspace.