Move away from autoconf interpolation to generate `ghc.cabal`

Currently configure is responsible for generating a number of .cabal files for the build system:

• compiler/ghc.cabal
• rts/rts.cabal
• ghc/ghc-bin.cabal
• utils/iserv/iserv.cabal
• utils/iserv-proxy/iserv-proxy.cabal
• utils/remote-iserv/remote-iserv.cabal
• utils/runghc/runghc.cabal utils/gen-dll/gen-dll.cabal
• libraries/ghc-boot/ghc-boot.cabal
• libraries/ghc-boot-th/ghc-boot-th.cabal
• libraries/ghci/ghci.cabal libraries/ghc-heap/ghc-heap.cabal
• libraries/libiserv/libiserv.cabal
• libraries/template-haskell/template-haskell.cabal

However, this is a severe nuisance since whenever you update the .cabal.in you need to rerun configure, which usually invalidates the vast majority of your already-built tree. We really should move the generation of these files into the build system. This would be a good thing to do after we finally drop the Make build system.