wasmtime: Add a Config::native_unwind_info method

[fitzgen]

This method configures whether native unwind information (e.g. .eh_frame on
Linux) is generated or not.

This helps integrate with third-party stack capturing tools, such as the system
unwinder or the backtrace crate. It does not affect whether Wasmtime can
capture stack traces in Wasm code that it is running or not.

Unwind info is always enabled on Windows, since the Windows ABI requires it.

This configuration option defaults to true.

Additionally, we deprecate Config::wasm_backtrace since we can always cheaply
capture stack traces ever since
#4431.

Fixes #4554

[fitzgen]

@peterhuene could you take a look at this and double check that the Windows-related bits make sense? Not 100% sure what I've said/configured is correct. Thanks!

[peterhuene]

Looks reasonable to me!

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