ada: Do not raise exceptions from Exp_Aggr.Packed_Array_Aggregate_Handled

An exception is now raised during bootstrap and this causes compatibility
issues with older compilers.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Packed_Array_Aggregate_Handled): Remove declaration
	and handler for Not_Handled local exception. Check the return value
	of Get_Component_Val instead.
	(Get_Component_Val): Return No_Uint instead of raising Not_Handled.