Adds rts/rts.cabal.in file

This is in preparation for cabalification of the `rts`. To be actually
able to parse this file, a rather recent Cabal is required. One after
commit 357d49d of haskell/cabal. The relevant PR to support the new
`asm-sources` and `cmm-sources` is haskell/cabal/pull/4857.

Not that this does *not* allow cabal to build the RTS. It does however
provide enough information such that cabal can `copy` and `register`
the package properly in the package database, if all the build
artifacts have been build properly.

As such it does not require any custom handling of the `rts` package.
As the rts as well as all the other packages built by the GHC built
system are built outside of cabal anyway.

Reviewers: bgamari, hvr, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, erikd

Differential Revision: https://phabricator.haskell.org/D4174

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