Hook up the -k option (that in the original GNU dlltool removes the @n suffix from the symbol that the final executable ends up linked to).
In llvm-dlltool, make sure that functions end up with the undecorate name type if this option is set and they are decorated. In mingw, when creating import libraries from def files instead of creating an import library as a side effect of linking a DLL, the symbol names in the def contain the stdcall/fastcall decoration (but no leading underscore).
By setting the undecorate name type, a linker linking to the import library will omit the decoration from the DLL import entry.
With this in place, mingw-w64 for i386 built with llvm-dlltool/clang produces import libraries that actually work.