Create a DisplayList benchmarks dylib on iOS

[gw280]

Currently we have no way to run the engine unittests or benchmarks on an iOS device, because the build outputs a binary which we cannot execute on iOS.

This is a quick way for us to be able to run benchmarks on an actual device:

• On iOS, instead of generating a benchmarking executable, build a dylib that's linked against libbenchmark.a
• In that dylib, don't depend on the existing //flutter/benchmarking code (which implements a main function), but instead depend on the //flutter/benchmarking:benchmarking_library dep, which exposes a RunBenchmarks(argc,argv) entrypoint.
• In the iOS test app, simply link against the generated libios_display_list_benchmarks.dylib library and call RunBenchmarks(argc, argv) in main and the benchmarks will run.

I will push the iOS test app in a future PR, but it is loosely modelled on the existing IosUnitTests app in //testing/ios.

flutter/flutter#96569

No new tests as this is the test code.

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• All existing and new tests are passing.

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[gaaclarke]

This is a little weird where if we are on iOS but don't have shell_enable_metal we get an empty source set. Seems like the conditional should be higher at the usage of the source set.

[gw280]

So this was just moved from inside the overall enable_unittests block into its own block that also allows for iOS, so it was already like that (except for the iOS condition).

I am a little wary of poking this because the large comment directly above the source_set suggests that there's some stuff going on here that's not entirely straightforward.