[lld][WebAssembly] Process stub libraries before performing LTO

There are cases where stub library processing can trigger new exports
which might require them to be included at LTO time.

Specifically `processStubLibraries` marks symbols as `forceExports`
which even effect the LTO process.

And since the LTO process can generate new undefined symbols
(specifically libcall function) we need to also process the stub
libraries after LTO.

Differential Revision: https://reviews.llvm.org/D147190