Duplicate page size determination in wasmtime-fiber

[alexcrichton]

Currently Wasmtime has a function crate::runtime::vm::host_page_size
but this isn't reachable from the wasmtime-fiber crate and instead tha
crate uses rustix::param::page_size to determine the host page size.
It looks like this usage of rustix is causing a panic in #10802.
Ideally wasmtime-fiber would be able to use the same function but the
crate separation does not currently make that feasible. For now
duplicate the logic of wasmtime into wasmtime-fiber as it's modest
enough to ensure that this does not panic.

Closes #10802

[alexcrichton]

I don't believe the CI failure is easily addressable with the previous design so I opted to duplicate the definition of host_page_size from the wasmtime crate into the wasmtime-fiber crate instead of passing it in as a parameter. That I think is regrettable duplication but "at least it's now commented duplication" and also fixes the panic for now.

I've also re-rolled review since Pat is on vacation right now since this has now changed substantially enough that I don't want to carry over the previous approval.

[alexcrichton]

I'm actually going to go ahead and flag this for merge. Apparently using this syscall prevents using rr with Wasmtime (rr crashes locally) so I'd like to get that fixed. If others have further comments on this I'm happy to handle them in follow-ups.