using a "gcc -m64" and a working ghc-7.8.0.20140228-i386-unknown-solaris2 I was able to create a 64Bit ghc-stage2 compiler that can compile a simple hello world program but may create binaries from larger sources that seg-fault. In particular "ghc-stage2 --interactive" seg-faults:

[Thread debugging using libthread_db enabled]
[New Thread 1 (LWP 1)]
[New LWP    2        ]
[New LWP    3        ]
[New LWP    4        ]
GHCi, version 7.8.0.20140228: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-simple ... linking ... done.
Loading package base ... linking ... 
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 1 (LWP 1)]
0x00000000041ea4c0 in ?? ()
(gdb) bt
#0  0x00000000041ea4c0 in ?? ()
#1  0x000000000369cf92 in resolveObjs ()
#2  0x000000000206de99 in cplU_info ()
#3  0x000000000000000c in ?? ()
#4  0x0000000003d0fe09 in base_GHCziEventziManager_zdLr88clvl8_closure ()
#5  0x0000000000000000 in ?? ()

The content of my mk/build.mk is just

INTEGER_LIBRARY = integer-simple

Creating a binary-dist also worked, but unpacking and "gmake install" failed at first because old-time, haskell98 and haskell2010 libraries were missing due to "!CrossCompiling" in ghc.mk and the second time with:

Reading package info from "rts/dist/package.conf.install" ... done.
"utils/ghc-cabal/dist-install/build/tmp/ghc-cabal-bindist" register libraries/ghc-prim dist-install "/local/home/maeder/haskell/ghc-7.8-x64-static/lib/ghc-7.8.0.20140228/bin/ghc" "/local/home/maeder/haskell/ghc-7.8-x64-static/lib/ghc-7.8.0.20140228/bin/ghc-pkg" "/local/home/maeder/haskell/ghc-7.8-x64-static/lib/ghc-7.8.0.20140228" '' '/local/home/maeder/haskell/ghc-7.8-x64-static' '/local/home/maeder/haskell/ghc-7.8-x64-static/lib/ghc-7.8.0.20140228' '/local/home/maeder/haskell/ghc-7.8-x64-static/share/doc/ghc/html/libraries' NO  
ghc-cabal: Bad interface file: dist-install/build/GHC/CString.hi
magic number mismatch: old/corrupt interface file? (wanted 129742, got
33214052)
gmake[1]: *** [install_packages] Fehler 1

What am I doing wrong for cross compiling? There is also #8378 using a different build host. #8713 also seem to use x86_64 solaris2 libs.