Add support for the recently added 128bits integer support in
Big_Integers.To_Big_Integer.

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

gcc/ada/

	* libgnat/s-genbig.ads, libgnat/s-genbig.adb (To_Bignum): New
	variant taking an Unsigned_128.
	* libgnat/a-nbnbin.adb (To_Big_Integer): Add support for 128
	bits signed and unsigned types.