gh-84461: Fix Emscripten umask and permission issues (GH-94002)

- Emscripten's default umask is too strict, see
  https://github.com/emscripten-core/emscripten/issues/17269
- getuid/getgid and geteuid/getegid are stubs that always return 0
  (root). Disable effective uid/gid syscalls and fix tests that use
  chmod() current user.
- Cannot drop X bit from directory.