Wasmtime debugging: clone only `.text`, not whole code image

In #12051 we added functionality to create private `CodeMemory`s per `Store` for each module instantiated in that store so that we could patch in breakpoints.

The functionality currently clones the whole `MmapVec` per store when guest-debugging is enabled. In theory we only need to clone the `.text` section. The clone is often cheap (when the file is mmap'd from disk) but not always, e.g. when compiled in-memory.

We should clone only `.text` and separate out the cloned `.text` in the representation of `StoreCodeStorage::Private`.