FFI asynchronous callback samples

[dcharkes]

We might want to add a sample for asynchronous callbacks to this repo. This requires compiling against dart_api(_dl).h.

See the samples in the SDK repo:

• Dart code: https://github.com/dart-lang/sdk/tree/master/samples/ffi/async
• C code: https://github.com/dart-lang/sdk/blob/master/runtime/bin/ffi_test/ffi_test_functions_vmspecific.cc

Context: dart-lang/sdk#46135