The version of Ada.Numerics.Big_Numbers.Big_Integers used in the light
and embedded runtimes is only meant for proof, not execution. As a
result, all subprograms were previously marked as imported, but this
leads to a spurious compilation error in GNAT. Work around that bug for
now by providing a dummy body for the generics inside that unit.

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

gcc/ada/

	* libgnat/a-nbnbin__ghost.adb (Signed_Conversions,
	Unsigned_Conversions): Mark subprograms as not imported.
	* libgnat/a-nbnbin__ghost.ads: Provide a dummy body.