Ada: Fix couple of oversights in Big_Integer package

Having too many special cases can be counter-productive as shown here.

gcc/ada/
	PR ada/125695
	* libgnat/s-genbig.adb ("**"): Do not drop the sign on the floor.
	(Big_Exp): Take into account the parity of the exponent for -2.

gcc/testsuite/
	* gnat.dg/bigint1.adb: New  test.