I'm cross-posting this to debian-sparc@l.d.o., maybe someone else here
has an idea how to build ghc for sparc64 without having to mess around
with cross-compilers on an amd64 host.
"Both ways require you to bootstrap from intermediate HC files: these are
the stylised C files generated by GHC when it compiles Haskell source.
Basically the idea is to take the HC files for GHC itself to the target
machine and compile them with gcc to get a working GHC, and go from
there."
So basically, hack a working compiler and then rebuild the package cleanly...