[WebAssembly] MC: Fix crash when -g specified.

At this point we don't output any debug sections or thier
relocations.

Differential Revision: https://reviews.llvm.org/D39076

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316240 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed