This finishes what #5529 started by exporting the constructors for
`IntPtr` and `WordPtr` from `Foreign.Ptr`, allowing them to be used in
`foreign` declarations.

Fixes #11983.

Test Plan: `make TEST=T11983`

Reviewers: simonpj, hvr, bgamari, austin

Reviewed By: simonpj

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D2142

GHC Trac Issues: #11983