Constant names of Big2xx32, Big2xx63 and Big2xx64 were misleading, as
SIngle_Size and Double_Size need not be 32 and 64. Replace by names that
don't refer to an explicit size.

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

gcc/ada/

	* libgnat/s-aridou.adb: Apply replacement.