[Bug ada/125695] Big_Exp special case for (+/-)2 ** K is broken for -2 and even K

[liam at liampwll dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125695

--- Comment #1 from Liam Powell <liam at liampwll dot com> ---
s/To_Big_Integer (-2) ** 1/To_Big_Integer (-3) ** 1