[lld][WebAssembly] Initial support for stub libraries

See the docs in lld/docs/WebAssembly.rst for more on this.

This feature unlocks a lot of simplification in the emscripten toolchain
since we can represent the JS libraries to wasm-ld as stub libraries.

See https://github.com/emscripten-core/emscripten/issues/18875

Differential Revision: https://reviews.llvm.org/D145308