[Ada] Ada2020: update Big_Numbers.* specs

[Ada] Ada2020: update Big_Numbers.* specs

2020-06-12  Bob Duff  <duff@adacore.com>

gcc/ada/

	* libgnat/a-nbnbin.adb, libgnat/a-nbnbin.ads,
	libgnat/a-nbnbin__gmp.adb, libgnat/a-nbnbre.adb,
	libgnat/a-nbnbre.ads: Update Put_Image, and uncomment the aspect
	specification.  Add pragmas Ada_2020.
	* libgnat/a-stouut.ads, libgnat/a-stteou.ads: Add Preelaborate,
	because the Big_Numbers packages have Preelaborate, and now
	depend on these Text_Output packages.