[lld][WebAssembly] -Bsymbolic creates indirect function table if needed

It can be that while processing relocations, we realize that in the end
we need an indirect function table.  Ensure that one is present, in that
case, to avoid writing invalid object files.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49397.

Differential Revision: https://reviews.llvm.org/D97843