[Ada] Code cleanups in System.Atomic_Counters

[Pierre-Marie de Rodat]

In particular, now that we are using Atomic_Unsigned which is marked
Atomic, we no longer need to mark Atomic_Counter.Value explicitly
atomic.

We can also get rid of all uses of 'Unrestricted_Access

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

gcc/ada/

	* libgnat/s-atocou.ads, libgnat/s-atocou__builtin.adb: Code
	cleanups.
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