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.