This replaces the special case done in "**" so far.

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

gcc/ada/

	* libgnat/s-genbig.adb ("**"): Remove capacity limit check.
	Improve code by using an extended return.
	(Normalize): Perform capacity limit check here instead which is
	the centralized place where (potentially large) big integers are
	allocated.