[Ada] Facilitate proof of Overwrite in bounded strings library
Consistently use >= operator in both the code and contracts of
function/procedure Overwrite, to facilitate proof, instead of the strict
inequality > sometimes, as only New_Item remains in the result in the
case of equal size too.
gcc/ada/
* libgnat/a-strbou.ads (Overwrite): Switch to >= operator in
contracts.
* libgnat/a-strsup.adb (Super_Overwrite): Switch to >= operator
in code of procedure (function already uses it).
* libgnat/a-strsup.ads (Super_Overwrite): Switch to >= operator
in contracts.