Fix pointer tagging mistake

f9c6d53f led to #14036. The
problem turned out to be rather simple: the `obj` pointer was
being tagged using `obj + arity`. Because this is C, that's done
with *pointer arithmetic*, which is not at all what we want. Add
appropriate casts.

Reviewers: austin, bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #14036

Differential Revision: https://phabricator.haskell.org/D3983