ada: Accept aspect Always_Terminates without expression

ada: Accept aspect Always_Terminates without expression

The recently added aspect Always_Terminates is now accepted without
explicit boolean expression, where a missing expression implicitly means
True, similar to aspects Async_Readers, Async_Writers, etc.

gcc/ada/

	* aspects.adb
	(Base_Aspect): Fix layout.
	* aspects.ads
	(Aspect_Argument): Expression for Always_Terminates is optional.
	* sem_prag.adb
	(Analyze_Always_Terminates_In_Decl_Part): Only analyze expression when
	pragma argument is present.
	(Analyze_Pragma): Argument for Always_Terminates is optional; fix
	whitespace for Async_Readers.