Allow benchmarks on Native

[lppedd]

Benchmarking Native is not really important for us, but we might use it to allow Kotlin/Native devs to improve the generated code performance.

To run the benchmark use:

./gradlew :antlr-kotlin-benchmarks:{platform}Test

For example:

./gradlew :antlr-kotlin-benchmarks:mingwX64Test

[ftomassetti]

We may need to update the readme. If the text on benchmarks is becoming too long we could extract a separate document and refer to it from the README

[lppedd]

@ftomassetti I've updated the readme to include a sample command for Native, but the length hasn't changed much.

As long as the readme is correctly split in sections, it should be easily accessible.

[ftomassetti]

Perfect, thank you @lppedd