[Ada] Create explicit ghost mirror unit for big integers

[Ada] Create explicit ghost mirror unit for big integers

gcc/ada/

	* Makefile.rtl: Add unit.
	* libgnat/a-nbnbin__ghost.adb: Move...
	* libgnat/a-nbnbig.adb: ... here. Mark ghost as ignored.
	* libgnat/a-nbnbin__ghost.ads: Move...
	* libgnat/a-nbnbig.ads: ... here.  Add comment for purpose of
	this unit. Mark ghost as ignored.
	* libgnat/s-widthu.adb: Use new unit.
	* sem_aux.adb (First_Subtype): Adapt to the case of a ghost type
	whose freeze node is rewritten to a null statement.