[Ada] Use atomic builtins for atomic counters on x86 (32bit)

[Pierre-Marie de Rodat]

Remove the x86-specific (32bit) variant of System.Atomic_Counters,
because the __atomic builtins work fine on all x86 CPUs except for i386,
where they can be provided by libatomic (when compiling with
-march=i386).

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

gcc/ada/

	* Makefile.rtl (X86_TARGET_PAIRS): Use __builtin variant of
	System.Atomic_Counters.
	* libgnat/s-atocou__x86.adb: Remove.
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