Fix handling of Tunables on cross-compiles

[alexcrichton]

This commit fixes how Wasmtime handles Tunables when targetting non-host platforms (or namely platforms with different pointer widths). Previously the host's Tunables would always be used instead of the target's tunables which meant that modules couldn't be loaded on the other platform due to the host having differing tunables by default.

This commit updates tunables in wasmtime::Config to all be optional and loading the actual Tunables is deferred until the target is known during Engine-creation time.

[github-actions]

Subscribe to Label Action

cc @peterhuene, @saulecabrera

This issue or pull request has been labeled: "wasmtime:api", "wasmtime:config", "winch"

Thus the following users have been cc'd because of the following labels:

• peterhuene: wasmtime:api
• saulecabrera: winch

To subscribe or unsubscribe from this label, edit the .github/subscribe-to-label.json configuration file.

Learn more.

[github-actions]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• If you added a new Config method, you wrote extensive documentation for
  it.

  Our documentation should be of the following form:

  Short, simple summary sentence.
  
  More details. These details can be multiple paragraphs. There should be
  information about not just the method, but its parameters and results as
  well.
  
  Is this method fallible? If so, when can it return an error?
  
  Can this method panic? If so, when does it panic?
  
  # Example
  
  Optional example here.
  

• If you added a new Config method, or modified an existing one, you
  ensured that this configuration is exercised by the fuzz targets.

  For example, if you expose a new strategy for allocating the next instance
  slot inside the pooling allocator, you should ensure that at least one of our
  fuzz targets exercises that new strategy.

  Often, all that is required of you is to ensure that there is a knob for this
  configuration option in wasmtime_fuzzing::Config (or one
  of its nested structs).

  Rarely, this may require authoring a new fuzz target to specifically test this
  configuration. See our docs on fuzzing for more details.

• If you are enabling a configuration option by default, make sure that it
  has been fuzzed for at least two weeks before turning it on by default.

---

To modify this label's message, edit the .github/label-messager/wasmtime-config.md file.

To add new label messages or remove existing label messages, edit the
.github/label-messager.json configuration file.

Learn more.

[fitzgen]

Nice!