[kernel] Remove thread_data pointer from TCB

The TCB is always stored at a constant offset within the thread object.
So instead of carrying an extra pointer, just implement thread::from_tcb
to get the thread.

src/kernel/objects/thread.h

@@ -27,8 +27,6 @@ struct TCB
 
 	uint32_t time_left;
 	uint64_t last_ran;
-
-	thread *thread_data;
 };
 
 using tcb_list = kutil::linked_list<TCB>;
@@ -47,6 +45,9 @@ public:
 		none     = 0x00
 	};
 
+	/// Get the pointer to the thread object containing this TCB
+	static thread * from_tcb(TCB *tcb);
+
 	/// Get the `ready` state of the thread.
 	/// \returns True if the thread is ready to execute.
 	inline bool ready() const { return has_state(state::ready); }