[Ada] Accept explicit SPARK_Mode Auto as configuration pragma

[Ada] Accept explicit SPARK_Mode Auto as configuration pragma

An explicit value of Auto is now accepted for a configuration pragma
SPARK_Mode, as a way to exempt a unit from complete adherence to
SPARK rules when using a global configuration pragma file where
SPARK_Mode=>On is specified.

gcc/ada/

	* sem_prag.adb (Analyze_Pragma): Accept SPARK_Mode=>Auto as
	configuration pragma.
	(Get_SPARK_Mode): Make the value for Auto explicit.
	* snames.ads-tmpl (Name_Auto): Add name.