[WebAssembly] Add -u/--undefined argument handling

Adds a new argument to wasm-lld, `--undefined`, with
similar semantics to the ELF linker. It pulls in symbols
from files contained within a `.a` archive, forcing them
to be included even if the translation unit would not
otherwise be pulled in.

Patch by Nicholas Wilson

Differential Revision: https://reviews.llvm.org/D40724

llvm-svn: 320004