ada: Tweak Is_Predefined_File_Name

ada: Tweak Is_Predefined_File_Name

This patch extends the set of names recognized by
Is_Predefined_File_Name so that names that start with "a-" and are longer
than 12 characters are considered predefined.

This is motivated by the need to make gprname work with the in-tree
a-strunb__shared.ads file that uses GNAT syntax extensions but whose name
wasn't considered predefined before this patch.

gcc/ada/ChangeLog:

	* fname.adb (Is_Predefined_File_Name): Tweak.