x86_64-darwin: Disable LTO

LTO on x86_64-darwin appears to break the ability to catch exceptions
correctly (maybe just for exception types defined in different
libraries). This leads to many weird test failures,
e.g. https://hydra.nixos.org/build/286312387 and
https://hydra.nixos.org/build/286312341.

packaging/dependencies.nix

@@ -79,6 +79,8 @@ let
         !stdenv.hostPlatform.isWindows
         # build failure
         && !stdenv.hostPlatform.isStatic
+        # LTO breaks exception handling on x86-64-darwin.
+        && stdenv.system != "x86_64-darwin"
       ) ''
         case "$mesonBuildType" in
         release|minsize) appendToVar mesonFlags "-Db_lto=true"  ;;