ada: Prevent calculation of negative stack counts

Negative numbers of stack counts have no meaning.

gcc/ada/

	* lib.ads, lib.adb (Primary_Stack_Count, Sec_Stack_Count,
	Increment_Primary_Stack_Count, Increment_Sec_Stack_Count,
	Unit_Record): Stack counts are never negative.
	* ali.ads (Unit_Record): Likewise.
	* bindgen.adb (Num_Primary_Stacks, Num_Sec_Stacks): Likewise.
	* exp_ch3.adb (Count_Default_Sized_Task_Stacks): Likewise.
	* sem_util.ads, sem_util.adb (Number_Of_Elements_In_Array):
	Likewise.