Rollup merge of #152134 - hoodmane:emscripten-crt-static-allow-dylibs, r=petrochenkov

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.

Author: matthiaskrgr