darwin.libffi: drop x86_64-darwin support

pkgs/os-specific/darwin/by-name/li/libffi/package.nix

@@ -53,16 +53,14 @@ mkAppleDerivation (finalAttrs: {
 
   postConfigure = ''
     # Use Apple’s configuration instead of the one generated by the `configure` script.
-    cp darwin/include/fficonfig_${stdenv.hostPlatform.darwinArch}.h build/fficonfig.h
-    cp darwin/include/ffitarget_${
-      if stdenv.hostPlatform.isAarch64 then "arm64" else "x86"
-    }.h build/include/ffitarget.h
+    cp darwin/include/fficonfig_arm64.h build/fficonfig.h
+    cp darwin/include/ffitarget_arm64.h build/include/ffitarget.h
     # Use `macCatalyst` instead of `iosmac` to avoid errors due to invalid availability annotations.
     substitute darwin/include/ffi.h build/include/ffi.h \
       --replace-fail iosmac macCatalyst
   '';
 
-  postBuild = lib.optionalString stdenv.hostPlatform.isAarch64 ''
+  postBuild = ''
     $CC -Os -Wl,-allowable_client,! -Wl,-not_for_dyld_shared_cache -Wl,-no_compact_unwind \
       src/aarch64/trampoline.S -dynamiclib -o libffi-trampolines.dylib \
       -Iinclude -Ibuild -Ibuild/include \
@@ -75,7 +73,7 @@ mkAppleDerivation (finalAttrs: {
       ln -s "$dev/include" "$dev/include/ffi"
     ''
     # Install the trampoline dylib since it is build manually.
-    + lib.optionalString stdenv.hostPlatform.isAarch64 ''
+    + ''
       cp libffi-trampolines.dylib "$out/lib/libffi-trampolines.dylib"
     '';