[Ada] Add the System.Bitfield_Utils runtime unit

[Ada] Add the System.Bitfield_Utils runtime unit

2019-08-21  Bob Duff  <duff@adacore.com>

gcc/ada/

	* Makefile.rtl (GNATRTL_NONTASKING_OBJS): Add s-bitutil.o and
	s-biutin.o.
	* exp_ch5.adb (Expand_Assign_Array_Bitfield): New function to
	generate a call to Copy_Bitfield. This is disabled for now.
	(Expand_Assign_Array_Loop_Or_Bitfield): New function to decide
	whether to call Expand_Assign_Array_Bitfield.
	(Expand_Assign_Array): Call Expand_Assign_Array_Loop_Or_Bitfield
	instead of Expand_Assign_Array_Loop.
	* libgnat/s-bitfie.ads, libgnat/s-bituti.adb,
	libgnat/s-bituti.ads: New units.
	* rtsfind.ads: Add enum literals for accessing Copy_Bitfield.

From-SVN: r274785