dump.c: handle GV being really a ref to a CV

dump.c: handle GV being really a ref to a CV

RT #129285

These days a 'GV' can actually just be a ref to a CV when the only thing
that would be stored in the glob is a CV. Update S_do_op_dump_bar() to
handle this. Formerly it would trigger an assert on a non-threaded build.

In fact, incorporate the fixed logic into a static function,
S_gv_display(), that is shared by both S_do_op_dump_bar() and
Perl_debop(); so both

    perl -Dx

and

    perl -Dt

get the benefit.

Also for the -Dx case, make it display the raw address of the GV too.