[Patch, Fortran] Use -huge()-1 as most negative integer for maxval/maxloc (PR30512)

[François-Xavier Coudert]

> Also, I just noticed -- in your libgfortran patches, you end up using
> -HUGE-1 for both integers and reals.  I presume on reals that just ends
> up being the same as -HUGE?

Hum... sounds like a bad practice, even if it doesn't hurt right now :(

FX