[Ada] New procedure Register_Global_Unhandled_Action

[Ada] New procedure Register_Global_Unhandled_Action

2020-06-04  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* libgnat/a-exextr.adb (Global_Unhandled_Action): New global
	variable.
	(Notify_Exception): Take into account Global_Unhandled_Action
	and fix latent race condition.
	(Exception_Action): Mark Favor_Top_Level so that variables can
	be atomic.
	(Global_Action): Mark atomic to remove the need for a lock.
	* libgnat/g-excact.ads, libgnat/g-excact.adb
	(Register_Global_Unhandled_Action): New procedure.
	(Register_Global_Action): Remove lock.
	* libgnat/s-stalib.ads (Raise_Action): Mark Favor_Top_Level to
	be compatible with Exception_Action.
	* sem_warn.adb (Warn_On_Unreferenced_Entity): Fix logic wrt
	Volatile entities and entities with an address clause: the code
	did not match the comment/intent.