[lld][WebAssembly] Fix crash on undefined+weak function syms in LTO objects

Symbols from LTO objects don't contain Wasm signatures, but we need a
signature when we create undefined/stub functions for missing weakly
undefined symbols.

Luckily, after LTO, we know that symbols that are not referenced by a
regular object file must not be needed in the final output so there
is no need to generate undefined/stub function for them.

Differential Revision: https://reviews.llvm.org/D126554