ada: Replace "All" argument to Extensions_Allowed pragma with "All_Extensions"

ada: Replace "All" argument to Extensions_Allowed pragma with "All_Extensions"

The argument to pragma Extensions_Allowed to enable all extensions is
no longer "All", but instead "All_Extensions".

gcc/ada/

	* doc/gnat_rm/gnat_language_extensions.rst: Update documentation.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Update
	documentation.
	* errout.adb
	(Error_Msg_GNAT_Extension): Update error message text.
	* par-prag.adb: Update pragma parsing. This includes changing the
	the name of the Check_Arg_Is_On_Or_Off formal parameter All_OK_Too
	to All_Extensions_OK_Too.
	* sem_prag.adb (Analyze_Pragma): In analyzing an
	Extensions_Allowed pragma, replace uses of Name_All with
	Name_All_Extensions; update a comment to reflect this.
	* snames.ads-tmpl: Add Name_All_Extensions declaration.
	* gnat_rm.texi: Regenerate.