[Ada] Avoid gnatbind regression caused by Copy_Bitfield

[Ada] Avoid gnatbind regression caused by Copy_Bitfield

The recent Copy_Bitfield change caused gnatbind to change elaboration
order, causing different error messages.

2019-09-18  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_ch5.adb (Expand_Assign_Array_Loop_Or_Bitfield): Move call
	to RTE_Available later, so it doesn't disturb the elab order.
	The RE_Copy_Bitfield entity is defined in package
	System.Bitfields which has a dependency on package
	System.Bitfield_Utils, which has it its spec:

	   pragma Elaborate_Body;

	The query on RTE_Available forces loading and analyzing
	System.Bitfields and all its withed units.

From-SVN: r275866