Introduce OutputableP

Some types need a Platform value to be pretty-printed: CLabel, Cmm
types, instructions, etc.

Before this patch they had an Outputable instance and the Platform value
was obtained via sdocWithDynFlags. It meant that the *renderer* of the
SDoc was responsible of passing the appropriate Platform value (e.g. via
the DynFlags given to showSDoc).  It put the burden of passing the
Platform value on the renderer while the generator of the SDoc knows the
Platform it is generating the SDoc for and there is no point passing a
different Platform at rendering time.

With this patch, we introduce a new OutputableP class:

   class OutputableP a where
      pdoc :: Platform -> a -> SDoc

With this class we still have some polymorphism as we have with `ppr`
(i.e. we can use `pdoc` on a variety of types instead of having a
dedicated `pprXXX` function for each XXX type).

One step closer removing `sdocWithDynFlags` (#10143) and supporting
several platforms (#14335).

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