[Ada] Update comment on __atomic_compare_exchange in s-atomic_primitives

[Ada] Update comment on __atomic_compare_exchange in s-atomic_primitives

Remove mention of unavailability, long obsolete, and reword suggestion of use
to indicate that we might want to switch to an internal interface using them.
The current wording suggests just that we should bind the current
Sync_Compare_And_Swap Ada subprograms to __atomic_compare builtins instead of
__sync_compare, which would be highly confusing.

2018-05-31  Olivier Hainque  <hainque@adacore.com>

gcc/ada/

	* libgnat/s-atopri.ads: Update comment on __atomic_compare_exchange
	builtins.

From-SVN: r261008