Fix parsing export lists

We were accepting
    module ExportCommaComma (id, reverse,,) where
where only 1 trailing comma should be permitted.
Remove unused __2Int_encodeDouble
Expose __word_encode{Float,Double}; fixes integer-simple build
Add some extra debugging info
Fix a build problem with integer-simple

We were trying to look up the mkInteger Id before we'd compiled
the Integer modules. I'm not sure why this never showed up with
integer-gmp; possibly we just always got lucky with the build
order.
Fix build problem: Error: junk `.get_pc_thunk.bx' after expression; trac #7799

Affects i386 only. Some gcc's generate code containing __i686, and then
define that symbol to 1, which causes compilation to fail. We undef
the symbol to work around it.
We can't use Integer literals when compiling the integer-* package, either