[Ada] Implement -gnateb switch

[Ada] Implement -gnateb switch

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Describe -gnateb switch.
	* doc/gnat_ugn/the_gnat_compilation_model.rst: Mention -gnateb
	switch in configuration pragma files section.
	* gnat_ugn.texi: Regenerate.
	* lib-writ.adb (Write_ALI): Strip directories from configuration
	files path if needed.
	* opt.ads: Declare Config_Files_Store_Basename option.
	* par.adb (Par): Save configuration file checksum.
	* switch-c.adb (Scan_Front_End_Switches): Set
	Config_Files_Store_Basename true if -gnateb is present.