[native_toolchain_c] Add linking for iOS

[goderbauer]

Towards #1376

We were basically able re reuse all the macOS work from #2360 for iOS and just had to add iOS-specific Tests.

This PR also fixes an issue with the treeshaking tests, which were accidentally overwriting size results from other test cases. An expect has been added to guard against this issue in the future.

Hint for reviewers: treeshake_helper.dart is easier to review with github's Hide whitespace option turned on.

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:code_assets	0.19.4	already published at pub.dev
package:data_assets	0.19.1	already published at pub.dev
package:ffi	2.1.4	already published at pub.dev
package:ffigen	19.1.0-wip	WIP (no publish necessary)
package:hooks	0.19.5	already published at pub.dev
package:hooks_runner	0.21.0	already published at pub.dev
package:jni	0.14.2	already published at pub.dev
package:jnigen	0.15.0-wip	WIP (no publish necessary)
package:native_doc_dartifier	0.0.1-pre	already published at pub.dev
package:native_toolchain_c	0.16.5	ready to publish	native_toolchain_c-v0.16.5
package:objective_c	8.1.0-wip	WIP (no publish necessary)
package:swift2objc	0.0.1-wip	WIP (no publish necessary)
package:swiftgen	0.0.1-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

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Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/native_doc_dartifier/example/native_doc_dartifier_example.dart
pkgs/native_doc_dartifier/lib/native_doc_dartifier.dart
pkgs/native_doc_dartifier/lib/src/native_doc_dartifier_base.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[goderbauer]

Skipping the breaking-check because of #2361. Local run is clean:

No changes detected!
Version Check
New version is OK!

Old version: "0.16.4"
New version: "0.16.5"
Needed version: "0.16.4"

Got "0.16.5" which is >= "0.16.4" (no changes)

[coveralls]

coverage: 88.027%. remained the same
when pulling 3abe9e3 on goderbauer:ios-linker
into 92e243f on dart-lang:main.

[dcharkes]

Thanks @goderbauer 🙏

[dcharkes]

Intentional change?

[goderbauer]

Yeah, to make it match the other tests.

[dcharkes]

should this be targetOs == OS.android instead, with a ! on androidTargetNdkApi? Ditto for the other OSes.

[goderbauer]

Since we check at the top of the method that androidTargetNdkApi must be set iff targetOS is Android it shouldn't matter logically. I find the current way without the extra null check easier to read, which is why I opted for that one.