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This frees wORD_SIZE up to be moved out of HaskellConstants

Also moved the type definition into RegAlloc.Linear.State to de-orphan
the Monad instance.

This means that we get e.g.
    pc_OFFSET_stgEagerBlackholeInfo = -24
rather than
    pc_OFFSET_stgEagerBlackholeInfo = 18446744073709551592

It no longer generates anything

We now also generate nice wrappers for the platformConstants
methods. For now it's all commented out as the definitions
conflict with those in Constants.

It is only generated when mode is Gen_Header; i.e. it's not used
in the compiler, only the RTS.

We now generate a platformConstants file that we can read at runtime.

This means we only need to build one copy of the program, which
will make life simpler as I plan to add more variants.

I've switched to passing DynFlags rather than Platform, as (a) it's
simpler to not have to extract targetPlatform in so many places, and
(b) it may be useful to have DynFlags around in future.

We don't actually use it yet