Set crt_static_allow_dylibs to true for Emscripten target And add a test. This is followup work to rust-lang/rust#151704. It introduced a regression where cargo is now unwilling to build cdylibs for Emscripten because `crt_static_default` is `true` but `crt_static_allows_dylibs` is `false`. Unfortunately the added test does not fail without the change because the validation logic is in Cargo, not in rustc. But it's good to have some coverage of this anyways.