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.

I probably should have done prtest:full sooner but as can probably be seen there's a lot of fixup commits here to get tests passing on all platforms in CI and such. Nontrivial changes include:

• Validation of Tunables for execution at runtime is deferred until a module is loaded. This enables 32-bit platforms, which disable signals_based_traps, to compile for 64-bit platforms (e.g. the disas test suite)
• Auto-configuration of Tunables based on crate features is skipped if --target is passed. This fixes disas cross-compilation tests from 32-to-64-bit where 64-bit hosts expect signals-based-traps.
• Some standalone tests/*.rs files have moved from /tests/*.rs to /crates/wasmtime/tests/*.rs to be able to use the #[cfg] directives printed by crates/wasmtime/build.rs
• Default Tunables for Pulley targets have been auto-adjusted to assume no signals-based-traps and no memory reservation. This fixes --target pulley32 on 32-bit platforms where the second bullet above wasn't otherwise applying to auto-disable from the defaults.
• Stack overflow messages for overflowing the async stack no longer happen with Pulley. That's because we no longer (intentionally) install signal handlers.

Juggling compile options I feel still isn't great. Ideally everything "just works" but it's a narrow needle to thread trying to juggle different crate features and targetting different environments. Ideally we'd be able to query has_native_signals and has_virtual_memory based on target_lexicon::Triple, but that involves inferring cfg(unix) or cfg(windows) which is a bit of a stretch at this time. I've attempted to have at least a somewhat reasonable middle-ground in the meantime.