Copy callstack API

[g0djan]

New WAMR public API to copy runtime call stack frames.

CAUTION: this APIs is not thread safe, that's why it's hidden behind feature flag for now. If you need to call it from another thread ensure the passed exec_env is suspended.

Our use case

Sometimes WAMR runtime gets stuck in production and we have no data where in the code compiled to WASM it happens. We currently only track such situations in a separate native thread. To increase visibility into the problem we developed internal solution that requires presence of this API in WAMR. If a separate thread finds that the WASM VM thread has stuck, it interrupts it with a user defined signal and calls this API to collect callstack. The main complexity is maintaining async-signal-safety and avoiding segfaults. For that we're maintaining atomic copies of exec_env, exec_env->module_inst, exec_env->module_inst->module. Those copies are always set to NULL before the referenced memory is freed. Before a call to this API those copies are always checked for validity. In our use case scenario we guarantee ourselves only absence of crashes but we realize that the frame data that we collect might be invalidated due to a signal interruption. However it's highly unlikely and is not a concern for us.

Have we tried existing WAMR APIs for our usecase?

Yes, we've tried suggested by maintainers wasm_cluster_suspend_thread and wasm_runtime_terminate.

1. In our production runtime often recovers from being stuck, so wasm_runtime_terminate is not a good option for us to report the call stack
2. The wasm_cluster_suspend_thread doesn't suit us either. Even if it did we'd still need API to iterate over stackframes.