onnxruntime: fix aarch64-darwin

...following cbourjau s idea

pkgs/development/libraries/eigen/default.nix

@@ -1,6 +1,7 @@
 { lib
 , stdenv
 , fetchFromGitLab
+, fetchpatch
 , cmake
 }:
 
@@ -17,6 +18,20 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./include-dir.patch
+
+    # Fixes e.g. onnxruntime on aarch64-darwin:
+    # https://hydra.nixos.org/build/248915128/nixlog/1,
+    # originally suggested in https://github.com/NixOS/nixpkgs/pull/258392.
+    #
+    # The patch is from
+    # ["Fix vectorized reductions for Eigen::half"](https://gitlab.com/libeigen/eigen/-/merge_requests/699)
+    # which is two years old,
+    # but Eigen hasn't had a release in two years either:
+    # https://gitlab.com/libeigen/eigen/-/issues/2699.
+    (fetchpatch {
+      url = "https://gitlab.com/libeigen/eigen/-/commit/d0e3791b1a0e2db9edd5f1d1befdb2ac5a40efe0.patch";
+      hash = "sha256-8qiNpuYehnoiGiqy0c3Mcb45pwrmc6W4rzCxoLDSvj0=";
+    })
   ];
 
   nativeBuildInputs = [ cmake ];