Kernel: Port process thread lists to SpinLockProtectedValue

I had to move the thread list out of the protected base area of Process
so that it could live with its lock (which needs to be mutable).
Ideally it would live in the protected area, so maybe we can figure out
a way to do that later.

Kernel/Thread.h

@@ -1372,6 +1372,9 @@ private:
     void yield_and_release_relock_big_lock();
     void yield_assuming_not_holding_big_lock();
     void drop_thread_count(bool);
+
+public:
+    using ListInProcess = IntrusiveList<Thread, RawPtr<Thread>, &Thread::m_process_thread_list_node>;
 };
 
 AK_ENUM_BITWISE_OPERATORS(Thread::FileBlocker::BlockFlags);