[lld][WebAssembly] Honor --allow-undefined for data symbols too
This was originally the way this worked before before
In retrospect it seems inconsistent that --allow-undefined doesn't
work for all symbols. See:
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