gcc-4.8: Get rid of lib64

Having a separate lib64 is bad because it requires special-casing in
lots of places.

Previously done in e4a11b4d92, which
apparently got lost going from 4.6 to 4.8.

pkgs/development/compilers/gcc/4.8/builder.sh

@@ -200,6 +200,15 @@ postConfigure() {
 }
 
 
+preInstall() {
+    # Make ‘lib64’ a symlink to ‘lib’.
+    if [ -n "$is64bit" -a -z "$enableMultilib" ]; then
+        mkdir -p $out/lib
+        ln -s lib $out/lib64
+    fi
+}
+
+
 postInstall() {
     # Remove precompiled headers for now.  They are very big and
     # probably not very useful yet.

pkgs/development/compilers/gcc/4.8/default.nix

@@ -504,6 +504,8 @@ stdenv.mkDerivation ({
 
   inherit enableParallelBuilding;
 
+  inherit (stdenv) is64bit;
+
   meta = {
     homepage = http://gcc.gnu.org/;
     license = "GPLv3+";  # runtime support libraries are typically LGPLv3+