PathScale Fortran mangles module symbols as "MY_SUB.in.MY_MODULE" and
also requires "my_module_" when the module is imported.  We cannot
provide the symbol with ".in." mangling so we should not provide
"my_module_" because it would duplicate the one in the Fortran-provided
object file.

Commit "FortranCInterface: Fix PathScale detection" (2010-01-22) already
made the same fix for the non-underscore module case.