Emscripten dynamic linking and libc

[hoodmane]

When dynamically linking Emscripten modules, Rust passes -lc to Emscripten. This causes wasm-ld: error: unable to find library -lc-debug. I see that -lc is coming via EmLinker.link_dylib. If I make link_dylib into a no-op this fixes the problem, though maybe that has other bad side effects. I am not sure how to filter out libraries that shouldn't be statically linked into a dynamic library. But it would be great to fix this. I am currently using an emcc linker wrapper that deletes the -lc argument.

@sbc100
xref emscripten-core/emscripten#17191

[hoodmane]

For a reproduction:

touch lib.rs
rustc lib.rs -C link-arg=-sSIDE_MODULE=2 --target wasm32-unknown-emscripten --crate-type cdylib

using tip of tree emcc and nightly rust.

[hoodmane]

Closed because the Emscripten issue that causes this problem seems to have been resolved.