Kernel: Make Thread::current smp-safe

Change Thread::current to be a static function and read using the fs
register, which eliminates a window between Processor::current()
returning and calling a function on it, which can trigger preemption
and a move to a different processor, which then causes operating
on the wrong object.

Kernel/Scheduler.cpp

@@ -483,7 +483,7 @@ void Scheduler::timer_tick(const RegisterState& regs)
     ASSERT_INTERRUPTS_DISABLED();
     ASSERT(Processor::current().in_irq());
 
-    auto current_thread = Processor::current().current_thread();
+    auto current_thread = Processor::current_thread();
     if (!current_thread)
         return;