Add a new unbounded functional sequence. This sequence is indexed by
Big_Positive and so is unbounded from the user and spark points view.
Hower the actually implemented sequence are bounded by Count_Type'Last.

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

gcc/ada/

	* libgnat/a-cfinse.adb, libgnat/a-cfinse.ads: Implementation
	files of the sequence.
	* Makefile.rtl, impunit.adb: Take into account the add of the
	new files