[WebAssembly] MC: Fix references to undefined externals in data section

Undefined externals don't need to have a size or an offset.
This was broken by r303915.  Added a test for this case.

This fixes the "Compile LLVM Torture (o)" step on the wasm
waterfall.

Differential Revision: https://reviews.llvm.org/D33803

llvm-svn: 304505