Avoid spurious rebuild actions in ada/gen_il

This uses the same mechanism as for ada/snames and ada/sdefault to avoid
spurious rebuild actions in the ada/gen_il directory.  This also avoids
copying some files into the generated directory, which is unnecessary.

gcc/ada/
	* Make-generated.in (do_gen_il): Replace with...
	(ada/stamp-gen_il): ...this.  Do not copy files into generated/.