Ignore async_stack_size if async_support is disabled

[guregu]

Fixes an issue where max_wasm_stack was being validated against async_stack_size when async_support was disabled, discussed in #6762.

I added a really simple test. I wasn't sure where to put it, let me know if there's a better spot for it (or some way to avoid the magic number of 8MB).

Background: wasmtime-go is currently panicking when trying to set the stack size (see: bytecodealliance/wasmtime-go#182).

[alexcrichton]

Thanks!

[fitzgen]

Thanks!

[guregu]

Thanks for the quick review and help with this, much appreciated! ❤️

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