[lld][WebAssembly] Honor --allow-undefined for data symbols too
This was originally the way this worked before before
https://reviews.llvm.org/D60882.
In retrospect it seems inconsistent that `--allow-undefined` doesn't
work for all symbols. See:
https://groups.google.com/g/emscripten-discuss/c/HSRgQiIq1gI/m/Kt9oFWHiAwAJ
I'm also planning a followup change which implement the full
`--unresolved-symbols=..` flags supported by ELF linkers (both ld and
ld.lld) since it seems more standard.
Differential Revision: https://reviews.llvm.org/D79247