[WebAssembly] Enabled -triple=wasm32-unknown-unknown-wasm path using ELF directive parser.

This is a temporary solution until a proper WASM implementation of
MCAsmParserExtension is in place, but at least for now will unblock this
path.

Added test to make sure this path works with the WASM Assembler.

Patch By Wouter van Oortmerssen!

Differential Revision: https://reviews.llvm.org/D45386

llvm-svn: 330370