Fix wasmtime epoch_interruption

[rennergade]

Our current idea for handling signals involves using the epoch_interruption feature of wasmtime to asynchronously interrupt code. As of the last time I checked in with @qianxichen233 this feature wasn't working anymore, possibly due to asyncify. Let's investigate the causes here and see if we can get interruption to work.

[qianxichen233]

Previously, the issue is about when epoch_interruption is enabled, new wasm instances created by fork/thread will be interrupted immediately once they start the execution (but the initial wasm instance will not). I highly suspect that this was just due to some incorrect handling of epoch initialization for new wasm instances, and I just confirmed this and fixed it - I only need to add one line of code when initializating new wasm instances for fork/thread and it could work.
Based on this, I also start experimenting signals with epoch_interruption. I did a minimal prototype for signal implementation, and I am able to run a process that stuck in a dead loop, but are able to jump to a custom user function that are able to do things like printf when host intended to trigger it.
However, fork is still not able to run within the custom user function (or signal handler). When I am trying to do fork within custom user function, I am getting unreachable instruction executed error, which is something I am expecting because I believe fork within signal handler shouldn't work correctly with current approach.