> BTW: I think, we should include mangle.c and cppmangle.c into glue layer (not frontend).

This is already handled by the glue layer without adding any useless layers on top of the current process.

> If someone will start to create back-end for any strange system with strange mangling of C function (@Symbol@), he should to define special С mangle class and provide this special mangle for front-end.

The compiler front end / language should not know or *care* about this.

> Not this code will be fail in, fro example, windows.

That could be a thrown back as a bug in pragma(mangle) then, or just how the dmd backend handles symbols declarated with pragma(mangle).