[gfortran,patch] Add -fmodule-private option (PR 31629)

[Paul Thomas]

FX,

:REVIEWMAIL:

OK

Thanks

Paul
> The attached simple patch adds a -fmodule-private option to make 
> module entities private as default, i.e. when you USE a module and 
> compile with this option, you cannot refer to any variable that wasn't 
> explicitly declared as PUBLIC. This is a tool and help for some 
> people's coding style.
>
> It's very simple as the code was already factored nicely, so it's in 
> fact a one-liner (plus option-handling stuff, documentation and 
> testcase). Bootstrapped and regtested on x86_64-linux, OK to commit?
>
> FX
>
>
>
> :ADDPATCH fortran: