[lld][WebAssembly] Avoid importing/exporting hidden symbols in shared libraries

We have some special handling for weakly defined symbols where we both
import and export them, but this is not needed for hidden symbols which
should never be imported or exported.

See https://github.com/emscripten-core/emscripten/pull/16972

This should also help with:
https://github.com/emscripten-core/emscripten/issues/15487

Differential Revision: https://reviews.llvm.org/D126491