[Ada] Improve efficiency of copying bit-packed slices

[Ada] Improve efficiency of copying bit-packed slices

This patch substantially improves the efficiency of copying large slices
of bit-packed arrays, by copying 32 bits at a time instead of 1 at a
time.

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

gcc/ada/

	* exp_ch5.adb (Expand_Assign_Array_Loop_Or_Bitfield): The call
	to Copy_Bitfield is now enabled.
	(Expand_Assign_Array_Bitfield): Multiply 'Length times
	'Component_Size "by hand" instead of using 'Size.

From-SVN: r275855