Be prepared for more aggregates in gigi

Be prepared for more aggregates in gigi

This makes sure that gigi is prepared to handle more aggregates in the
special memset code path.

	* sem_aggr.ads (Is_Single_Aggregate): New function.
	* sem_aggr.adb (Is_Others_Aggregate): Use local variable.
	(Is_Single_Aggregate): New function to recognize an aggregate with
	a single association containing a single choice.
	* fe.h (Is_Others_Aggregate): Delete.
	(Is_Single_Aggregate): New declaration.
	* gcc-interface/trans.c (gnat_to_gnu) <N_Assignment_Statement>: Call
	Is_Single_Aggregate instead of Is_Others_Aggregate.