[WebAssembly] MC: Use zero for provisional value of undefined symbols

This is more in line with what happens in the final
executable when symbols are undefined (i.e. weak
references).

Differential Revision: https://reviews.llvm.org/D41840

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322130 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed