filesystem: Paper over fadvise harmless failure on macos

[wingo]

On macos and ios, fadvise is only implemented for WillNeed and dispatches via the system-interface crate to the F_RDADVISE fcntl. If you call WillNeed on an out-of-bounds offset, this fcntl returns FileTooLarge. Here we paper over this harmless error code to avoid this needless platform-dependent nonuniformity. Should fix wasmtime for https://github.com/WebAssembly/wasi-testsuite/issues/178.