fix: Base.GMP.MPZ.invert yielding return code instead of return value (#56894)
There is a bug in `Base.GMP.MPZ.invert` it returned GMP return code,
instead of the actual value. This commit fixes it.
Before:
```
julia> Base.GMP.MPZ.invert(big"3", big"7")
1
```
After:
```
julia> Base.GMP.MPZ.invert(big"3", big"7")
5
```