Don't build abi_check for crosses

[Benjamin Kosnik]

># See the comment in libstdc++-v3/src/Makefile.am.  GCC can compile
># C++ files, and this prevents libtool trying to link libstdc++ with
># -lstdc++.  Only a problem in host-x-host builds.
># Note that we must explicitly add -shared-libgcc or libstdc++ won't
># be linked to libgcc_s.so.
>export CXX_FOR_TARGET="$CC -shared-libgcc"
>export CXX="$CC -shared-libgcc"
>
>The problem is, setting CXX and CXX_FOR_TARGET in the environment
>overrides the way the toplevel passes them to libstdc++. 
>testsuite_flags.in recovers a real CXX:
>      CXX=`echo "$CXX_build" | sed 's,gcc/xgcc ,gcc/g++ ,'`
>but that's hardly right if CXX was specified in the environment.

Agggh. I see.

I was hoping some of the magic and goo associated with CXX and CXXFLAGS
setting and saving in the libstdc++ Makefiles would go away, but that's
beyond the scope of what you are trying to do.

-benjamin