After some testing with an ultrasparc cross-compiler:

Updating the cross-build expressions, adding some flexibility.
Updated the linux headers used cross building, as 2.6.28 had bugs on endianness in
sparc64.
There were, as usual some bugs in gcc. Maybe not many make a cross compiler to
ultrasparc.

For the record, I could build an ultrasparc kernel with this base nix:
import /etc/nixos/nixpkgs/default.nix   # The root nixpkgs default.nix
{
    crossSystem = {
        config = "sparc64-unknown-linux";
        bigEndian = true;
        arch = "sparc64";
        float = "soft";
        withTLS = true;
        cpu = "ultrasparc";
    };

    config = pkgs: {
      packageOverrides = pkgs : {
        platform = {
            name = "sparc64";
            kernelHeadersBaseConfig = "sparc64_defconfig";
            kernelBaseConfig = "sparc64_defconfig";
            kernelArch = "sparc";
            kernelAutoModules = false;
            kernelTarget = "zImage";
            uboot = null;
        };
      };
    };
}

Although it did not boot directly in qemu-system-sparc64:
[sparc64] Kernel already loaded
Unhandled Exception 0x0000000000000020
PC = 0x0000000000404000 NPC = 0x0000000000404004

svn path=/nixpkgs/trunk/; revision=20269

pkgs/development/compilers/gcc-4.4/default.nix

@@ -57,8 +57,11 @@ let version = "4.4.3";
 
     javaAwtGtk = langJava && gtk != null;
 
+    withCPU = if cross ? cpu then " --with-cpu=${cross.cpu}" else "";
+
     crossConfigureFlags =
       "--target=${cross.config}" +
+      withCPU +
       (if crossStageStatic then
         " --disable-libssp --disable-nls" +
         " --without-headers" +

pkgs/development/libraries/glibc-2.11/common.nix

@@ -67,9 +67,9 @@ stdenv.mkDerivation ({
      then "--enable-profile"
      else "--disable-profile")
   ] ++ stdenv.lib.optionals (cross != null) [
-    "--with-tls"
+    (if cross.withTLS then "--with-tls" else "--without-tls")
+    (if cross.float == "float" then "--without-fp" else "--with-fp")
     "--enable-kernel=2.6.0"
-    "--without-fp"
     "--with-__thread"
   ] ++ stdenv.lib.optionals (stdenv.system == "armv5tel-linux") [
     "--host=arm-linux-gnueabi"

pkgs/development/libraries/glibc-2.11/default.nix

@@ -46,6 +46,8 @@ in
         libc_cv_forced_unwind=yes
         libc_cv_c_cleanup=yes
         libc_cv_gnu89_inline=yes
+        # Only due to a problem in gcc configure scripts:
+        libc_cv_sparc64_tls=${if cross.withTLS then "yes" else "no"}
         EOF
         export BUILD_CC=gcc
         export CC="$crossConfig-gcc"

pkgs/os-specific/linux/kernel-headers/2.6.32.nix

@@ -1,21 +1,21 @@
-{stdenv, fetchurl, perl, cross ? null}:
+{stdenv, fetchurl, perl, cross ? null, platform}:
 
-assert stdenv.isLinux;
+assert cross == null -> stdenv.isLinux;
 
-let version = "2.6.32.4"; in
+let version = "2.6.32.9"; in
 
 stdenv.mkDerivation {
   name = "linux-headers-${version}";
 
   src = fetchurl {
     url = "mirror://kernel/linux/kernel/v2.6/linux-${version}.tar.bz2";
-    sha256 = "1n8pj05sazxv1dgi68q61lrvrnzvvx61qqw6kx80vqizqanz97z1";
+    sha256 = "1g6hs7j5kmifb3phbnckdmrnxd0cpqrijnnbry86z26npsh9my7l";
   };
 
   targetConfig = if (cross != null) then cross.config else null;
 
   platform =
-    if cross != null then cross.arch else
+    if cross != null then platform.kernelArch else
     if stdenv.system == "i686-linux" then "i386" else
     if stdenv.system == "x86_64-linux" then "x86_64" else
     if stdenv.system == "powerpc-linux" then "powerpc" else
@@ -33,6 +33,7 @@ stdenv.mkDerivation {
     if test -n "$targetConfig"; then
        export ARCH=$platform
     fi
+    make ${platform.kernelHeadersBaseConfig}
     make mrproper headers_check
   '';
 

pkgs/top-level/all-packages.nix

@@ -1967,7 +1967,9 @@ let
     binutilsCross = binutilsCross cross;
     libcCross = libcCross cross;
     profiledCompiler = false;
-    enableMultilib = true;
+    enableMultilib = false;
+    # cross-building for ultrasparc in 4.4.3 will require disabling shared due to a gcc bug.
+    # enableShared = false;
     crossStageStatic = false;
   };
 
@@ -1976,6 +1978,7 @@ let
         crossStageStatic = true;
         langCC = false;
         libcCross = null;
+        enableShared = true;
     });
     libc = null;
     binutils = binutilsCross cross;
@@ -5841,8 +5844,8 @@ let
 
   linuxHeaders = linuxHeaders_2_6_28;
 
-  linuxHeadersCross = cross : forceBuildDrv (import ../os-specific/linux/kernel-headers/2.6.28.nix {
-    inherit stdenv fetchurl cross perl;
+  linuxHeadersCross = cross : forceBuildDrv (import ../os-specific/linux/kernel-headers/2.6.32.nix {
+    inherit stdenv fetchurl cross perl platform;
   });
 
   linuxHeaders_2_6_18 = import ../os-specific/linux/kernel-headers/2.6.18.5.nix {

pkgs/top-level/release.nix

@@ -708,6 +708,7 @@ let
       bigEndian = false;
       arch = "arm";
       float = "soft";
+      withTLS = true;
   };
   nativePlatforms = linux;
 in {