Routine Lock_Free_Try_Write was using deprecated __sync GCC builtins.
Now it uses __atomic builtins, which are recommended for new code.

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

gcc/ada/

	* libgnat/s-atopri.ads (Atomic_Compare_Exchange): Replaces
	deprecated Sync_Compare_And_Swap.
	* libgnat/s-atopri.adb (Lock_Free_Try_Write): Switch from __sync
	to __atomic builtins.