d: Prefix object files from the root package with 'root-'

d: Prefix object files from the root package with 'root-'

None of the front-end module names in either the dmd or root package
collide just yet, but that does not mean they won't in the future.

gcc/d/ChangeLog:

	* Make-lang.in (D_FRONTEND_OBJS): Prefix object files from the root
	package with root-.
	(d/root-%.o): New recipe.