Compiler aborts when the Restriction_Warning (No_Tasking) appears as a
configuration pragma, and the main unit includes a task declaration.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_prag.adb (Process_Restrictions_Or_Restriction_Warnings):
	when the restriction is a configuration pragma and specifies
	No_Tasking, a global flag is set to reject task declarations,
	and to prevent the construction of Master entities. The flag
	must not be set if the pragma is a Restriction_Warning, in which
	case task declarationns are allowed.