`.hspp` and `.hscpp` are haskell files that have already been preprocessed.

Treat `.hspp` and `.hscpp` as Haskellish sources again, as they were before
commit a10e1990. This way, ghc --make will load their imports.

Make sure that `.cmm` and `.cmmcpp` are still not treated as Haskellish,
by moving them out of `haskell_src_suffixes` (but still keeping them in
haskellish_suffixes, though I'm not sure what the purpose of that group
is).

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D778