[wasm] Cleanup native build parts of WasmApp.targets (#52732)

[wasm] Cleanup native build parts of WasmApp.targets (#52732)

* [wasm] Split _WasmNativeBuild into separate targets, as steps

- this gives us more control over the flow, and will be useful for
  dependency checking (future work)

* [wasm] WasmApp.targets: Decouple aot compilation from native relinking

* [wasm] Convert emcc flags from property to item, and use rsp files

The default arguments are still from `emcc-flags.txt`, which will get
converted into a rsp file and used with this, in a future PR.

* cleanup

* [wasm] Add new properties to explicitly control optimization level used

.. for compiling native files, and linking.

`$(WasmLinkOptimizationFlag)`, and `$(WasmCompileOptimizationFlag)`

note: right now `emcc-flags.txt` has flags already specified, but that
will go away in favor of only these flags (future work)

* [wasm] Add new properties to specify extra emcc flags

`$(EmccExtraLDFlags)`, `$(EmccExtraCFlags)`

* dummy