graphite2: fix cross-compilation

Cross compilation failed for me with the following message:
24/85 Test #48: charis7 ..........................   Passed    0.02 sec
      Start  1: nolib-stdc++-graphite2_core
Could not find executable readelf

In other words, the check failed because one of the tests uses readelf and when cross-compiling this tool has a prefix for the target architecture.

pkgs/development/libraries/silgraphite/graphite2.nix

@@ -28,6 +28,10 @@ stdenv.mkDerivation rec {
     #   https://github.com/silnrsi/graphite/pull/74
     substituteInPlace tests/CMakeLists.txt \
       --replace 'add_subdirectory(nametabletest)' '#add_subdirectory(nametabletest)'
+
+    # support cross-compilation by using target readelf binary:
+    substituteInPlace Graphite.cmake \
+      --replace 'readelf' "${stdenv.cc.targetPrefix}readelf"
   '';
 
   cmakeFlags = lib.optionals static [