Add x64 ddm variants (#166511)

As a follow up to https://github.com/flutter/flutter/pull/162855. Adding
x64 allows us to add tests internally.

Towards b/393719931

Tested:

```
$ bin/et build --config ci/android_release_arm64_ddm
$ bin/et build --config ci/android_release_x64_ddm
```

cc @sigmundch 

## Pre-launch Checklist

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there for submitting PRs.
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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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engine/src/flutter/ci/builders/linux_android_aot_engine_ddm.json

@@ -64,6 +64,52 @@
                     ]
                 }
             ]
+        },
+        {
+            "archives": [
+                {
+                    "name": "ci/android_release_x64_ddm",
+                    "type": "gcs",
+                    "base_path": "out/ci/android_release_x64_ddm/zip_archives/",
+                    "include_paths": [
+                        "out/ci/android_release_x64_ddm/zip_archives/android-x64-release-ddm/artifacts.zip",
+                        "out/ci/android_release_x64_ddm/zip_archives/android-x64-release-ddm/linux-x64.zip",
+                        "out/ci/android_release_x64_ddm/zip_archives/android-x64-release-ddm/symbols.zip",
+                        "out/ci/android_release_x64_ddm/zip_archives/android-x64-release-ddm/analyze-snapshot-linux-x64.zip"
+                    ],
+                    "realm": "production"
+                }
+            ],
+            "drone_dimensions": [
+                "device_type=none",
+                "os=Linux"
+            ],
+            "gclient_variables": {
+                "use_rbe": true
+            },
+            "gn": [
+                "--target-dir",
+                "ci/android_release_x64_ddm",
+                "--runtime-mode",
+                "release",
+                "--android",
+                "--android-cpu",
+                "x64",
+                "--no-prebuilt-dart-sdk",
+                "--gn-args=dart_dynamic_modules=true",
+                "--rbe",
+                "--no-goma"
+            ],
+            "name": "ci/android_release_x64_ddm",
+            "description": "Produces experimental release mode artifacts to target x64 Android from a Linux host with dynamic modules enabled.",
+            "ninja": {
+                "config": "ci/android_release_x64_ddm",
+                "targets": [
+                    "default",
+                    "clang_x64/gen_snapshot",
+                    "flutter/shell/platform/android:analyze_snapshot"
+                ]
+            }
         }
     ],
     "generators": {

engine/src/flutter/ci/builders/linux_android_debug_engine_ddm.json

@@ -47,6 +47,46 @@
                     "flutter"
                 ]
             }
+        },
+        {
+            "archives": [
+                {
+                    "name": "ci/android_debug_x64_ddm",
+                    "type": "gcs",
+                    "base_path": "out/ci/android_debug_x64_ddm/zip_archives/",
+                    "include_paths": [
+                        "out/ci/android_debug_x64_ddm/zip_archives/android-x64-ddm/artifacts.zip",
+                        "out/ci/android_debug_x64_ddm/zip_archives/android-x64-ddm/symbols.zip"
+                    ],
+                    "realm": "production"
+                }
+            ],
+            "drone_dimensions": [
+                "device_type=none",
+                "os=Linux"
+            ],
+            "gclient_variables": {
+                "use_rbe": true
+            },
+            "gn": [
+                "--target-dir",
+                "ci/android_debug_x64_ddm",
+                "--android",
+                "--android-cpu=x64",
+                "--no-lto",
+                "--no-prebuilt-dart-sdk",
+                "--gn-args=dart_dynamic_modules=true",
+                "--rbe",
+                "--no-goma"
+            ],
+            "name": "ci/android_debug_x64_ddm",
+            "description": "Produces experimental debug mode artifacts to target x64 Android from a Linux host with dynamic modules enabled.",
+            "ninja": {
+                "config": "ci/android_debug_x64_ddm",
+                "targets": [
+                    "flutter"
+                ]
+            }
         }
     ],
     "generators": {