A little help for clang++ in toke.c

This was failing to compile with Apple LLVM version 6.0 like so:

toke.c:6558:13: error: no member named 'Perl_rv2cv_op_cv' in 'gv';
                       did you mean simply 'Perl_rv2cv_op_cv'?
                                : (CV *)gv
                                        ^~
Make DB_File compile under clang++.

Follow-up to 7f15aad6d6ce55cb.  Also noted upstream at

https://rt.cpan.org/Ticket/Display.html?id=96126

the necessity of, when rolling one's own dNOOP, to do everything
the core dNOOP does.