[WebAssembly] Fix expected contents of relocations with addends

This fixes issues found on the wasm waterfall related to relocations
with addends. Undefined symbols, even those with addends should
always have a provisional value of zero. At least this is what llvm
emits (and I believe this is true for ELF too).

Differential Revision: https://reviews.llvm.org/D44451

llvm-svn: 327468