[WebAssembly] Add support for --gc-sections

In this initial version we only GC symbols with `hidden` visibility since
other symbols we export to the embedder.

We could potentially modify this the future and only use symbols
explicitly passed via `--export` as GC roots.

This version of the code only does GC of data and code. GC for the
types section is coming soon.

Differential Revision: https://reviews.llvm.org/D42511

llvm-svn: 323842