Kernel/SMP: Mark s_smp_enabled READONLY_AFTER_INIT

We can't enter/leave SMP mode once the kernel is up and running.

Kernel/Arch/x86/common/Processor.cpp

@@ -31,7 +31,7 @@ READONLY_AFTER_INIT FPUState Processor::s_clean_fpu_state;
 
 READONLY_AFTER_INIT static ProcessorContainer s_processors {};
 READONLY_AFTER_INIT Atomic<u32> Processor::g_total_processors;
-static volatile bool s_smp_enabled;
+READONLY_AFTER_INIT static volatile bool s_smp_enabled;
 
 static Atomic<ProcessorMessage*> s_message_pool;
 Atomic<u32> Processor::s_idle_cpu_mask { 0 };