[gfortran] Fix PR 31144: strengthen name mangling

[Tobias Schlüter <tobias dot schlueter at physik dot uni-muenchen dot de>]

Hi,

this one-liner changes our name-mangling to be more robust.  Before this 
patch a well chosen pair of module name and module procedure name could 
lead to a collision between the symbols in two different modules.  This 
is fixed by adding capital letters to the mangled symbol name.  In order 
to not start a bikeshed discussion, I'm using my dictatorial powers to 
force my choice of the letters 'M', O' and 'D' on you.  Just kidding, if 
there happens to be any reason to not use these letters, please let me 
know, and I'll change to my second choice of "G4_7r4N_Rv1z".

Testing is underway, I'll commit this under the obviously correct rule 
if I don't receive any objections.  I'm not sure if this is appropriate 
for 4.2, after all it will break backwards compatibility for 
user-compiled code.

Cheers,
- Tobi