[lld][WebAssembly] stub objects: Fix handling of LTO libcall dependencies

This actually simplifies the code by performs a pre-pass of the stub
objects prior to LTO.

This should be the final change needed before we can make the switch
on the emscripten side: https://github.com/emscripten-core/emscripten/pull/18905

Differential Revision: https://reviews.llvm.org/D148287