Subzero: Fix multiply defined symbols in Windows/g++ build. Based on build failure messages in https://build.chromium.org/p/tryserver.nacl/builders/nacl-toolchain-win7-pnacl-x86_64/builds/3609/steps/llvm_i686_w64_mingw32%20%28build%29/logs/stdio . BUG= none R=jpp@chromium.org Review URL: https://codereview.chromium.org/2601653002 .