Kernel: Clean up thread stacks when a thread dies

We were forgetting where we put the userspace thread stacks, so added a
member called Thread::m_userspace_thread_stack to keep track of it.

Then, in ~Thread(), we now deallocate the userspace, kernel and signal
stacks (if present.)

Out of curiosity, the "init_stage2" process doesn't have a kernel stack
which I found surprising. :^)

Kernel/Thread.h

@@ -326,6 +326,7 @@ private:
     u32 m_pending_signals { 0 };
     u32 m_signal_mask { 0 };
     u32 m_kernel_stack_base { 0 };
+    RefPtr<Region> m_userspace_stack_region;
     RefPtr<Region> m_kernel_stack_region;
     RefPtr<Region> m_kernel_stack_for_signal_handler_region;
     SignalActionData m_signal_action_data[32];