v.pref: allow for -os wasm32_emscripten and later filtering _d_wasm32_emscripten.c.v and _notd_wasm32_emscripten.c.v files.

v.pref: allow for -os wasm32_emscripten and later filtering _d_wasm32_emscripten.c.v and _notd_wasm32_emscripten.c.v files. #4951