[Ada] Ada2020: AI12-0055 No_Dynamic_CPU_Assignment restriction

[Ada] Ada2020: AI12-0055 No_Dynamic_CPU_Assignment restriction

gcc/ada/

	* libgnat/s-rident.ads (No_Dynamic_CPU_Assignment): New
	restriction. Add it to all relevant profiles.
	* sem_ch13.adb (Attribute_CPU): Check No_Dynamic_CPU_Assignment
	restriction.
	(Attribute_CPU, Attribute_Dispatching_Domain,
	Attribute_Interrupt_Priority): Remove error checks -- these are
	checked in the parser.
	* sem_prag.adb (Pragma_CPU): Check No_Dynamic_CPU_Assignment
	restriction.  We've got a little violation of DRY here.
	* sem.ads, sem_ch3.ads: Minor comment fix.