fix(turbopack): use file persistence on wasm
Compile AccessMode::Mmap and its code paths only on targets that support memory mapping. WASI now defaults structurally to the file-backed access mode added in #97873, allowing the three kv_backing_storage persistence tests to run instead of being skipped.
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
Co-authored-by: Luke Sandberg <210140+lukesandberg@users.noreply.github.com>