MemoryImageSlot doesn't handle the case where RuntimeLinearMemory::byte_size is not host page-aligned

Discovered this while working on #9652.

Looking at `LocalMemory::new`:

https://github.com/bytecodealliance/wasmtime/blob/bc656c72126ca3dcdc9477960b88a1816d2c09cb/crates/wasmtime/src/runtime/vm/memory.rs#L484-L487

It passes in `alloc.byte_size()` as the `accessible` parameter to `MemoryImageSlot::create`:

https://github.com/bytecodealliance/wasmtime/blob/bc656c72126ca3dcdc9477960b88a1816d2c09cb/crates/wasmtime/src/runtime/vm/cow.rs#L337-L345

`self.accessible` is not rounded up to the host page size and is instead stored directly. But other places assume that `self.accessible` is page-aligned, for example:

https://github.com/bytecodealliance/wasmtime/blob/bc656c72126ca3dcdc9477960b88a1816d2c09cb/crates/wasmtime/src/runtime/vm/cow.rs#L425

This ends up resolving to `mprotect`:

https://github.com/bytecodealliance/wasmtime/blob/bc656c72126ca3dcdc9477960b88a1816d2c09cb/crates/wasmtime/src/runtime/vm/sys/unix/vm.rs#L9-L11

`mprotect` requires that its address is page-aligned, and will produce an `EINVAL` if it isn't.

I think there are likely also places where it panics or possibly even causes UB.