[patch,gfortran] Fix PR17917 - module equivalences

[Paul THOMAS]

> Could you explain what this is for? I see we write it and read it,
> but it's not used for anything else. Needed for follow-up work?
> 

The module name is used to prevent multiple declarations of each union in the host scope.
As it stands, the code runs correctly but looks messy.  I'll implement what is
needed in trans-common.c(build_module_equivalences).

Paul T