Make the `NativeAssets` target consistent with `build_info.dart`'s
documentation on missing `IosArchs` or `DarwinArchs`.

Bug:

* https://github.com/flutter/flutter/issues/136931

Also, updates the doc to reflect that MacOS is by default built for both
x64 and arm64. The PR making universal binaries the default didn't
update the doc comment.

* https://github.com/flutter/flutter/pull/100271

Please note that these defines are handled inconsistently in
`flutter_tools`. In some places they default to what's specified in the
doc-comment. In other places a `MissingDefineException` is thrown. I
believe the code around `build_info.dart` and the `environment` could
benefit from a wrapping so that defaults or missing definitions are
handled consistently in the code base.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
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.
- [x] 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].
- [ ] All existing and new tests are passing.

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[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
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