Spec units for verification of the GNAT standard library with GNATprove
must be listed as part of the libgnat package, as otherwise libadalang
will complain about missing dependencies.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* Makefile.rtl (GNATRTL_NONTASKING_OBJS): Include
	System.Value_U_Spec and System.Value_I_Spec units.