[Ada] Ensure System.Tasking.Debug.Known_Tasks component access is atomic

[Pierre-Marie de Rodat]

Multiple threads can access the elements of
System.Tasking.Debug.Known_Tasks concurrently. While the compiler will
generally produce code that reads and writes Task_Ids atomically since
Task_Id is word size, it is best to be explicit to prevent data race
issues that may arise from non-atomic component accesses.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnarl/s-tasdeb.ads (Known_Tasks): Add Atomic_Components
	aspect.
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