Another patch extracted from !3367 (closed)
This patch removes the use of `sdocWithDynFlags` from GHC.CmmToAsm.*.Ppr
To do that I've had to make some refactoring:
* X86' and PPC's `Instr` are no longer `Outputable` as they require a
`Platform` argument
* `Instruction` class now exposes `pprInstr :: Platform -> instr -> SDoc`
* as a consequence, I've refactored some modules to avoid .hs-boot files
* added (derived) functor instances for some datatypes parametric in the
instruction type. It's useful for pretty-printing as we just have to
map `pprInstr` before pretty-printing the container datatype.