Provide a way to setup JVM args for a task generating JMH benchmarks

[fzhinkin]

There's no way to specify JVM args for JmhBytecodeGeneratorTask (or, to be more precise, for its worker). As a result, it's kinda problematic to benchmark JVM-specific code that requires additional flags to work (like, --enable-preview).

Currently, benchmark generation fails with something like:

Execution failed for task ':mainBenchmarkGenerate'.
> There was a failure while executing work items
   > A failure occurred while executing kotlinx.benchmark.gradle.JmhBytecodeGeneratorWorker
      > Preview features are not enabled for org/example/PreviewApiBenchmark (class file version 69.65535). Try running with '--enable-preview'