Deparse stash subs stores as RVs

Commit 2eaf799e74b stop most declarations like ‘sub foo{}’ from creat-
ing GVs.  Instead, simple sub refs are stored in the stash.

That broke deparsing of package subs:

$ perl5.21.4 -MO=Deparse -e 'sub foo{die}'
-e syntax OK

Deparse needs to know about these references, so that we get this:

$ ./perl -Ilib -MO=Deparse -e 'sub foo{die}'
sub foo {
    die;
}
-e syntax OK