exp_dbug.ads: Fix outdated description.

	* exp_dbug.ads: Fix outdated description.  Mention link between XVS
	and XVZ objects.
	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Record_Subtype>: Set
	XVZ variable as unit size of XVS type.
	(maybe_pad_type): Likewise.

From-SVN: r158703