[LTO/gold] Fix workaround for old plugin-api.h in --wrap support

The workaround for older plugin-api.h in r327506 unfortunately
used another union member that is also fairly new and not available
in the plugin-api.h on some of the bots, leading to:
http://lab.llvm.org:8011/builders/clang-with-thin-lto-ubuntu/builds/9121/steps/build-stage2-LLVMgold.so/logs/stdio

Change to use a different member that we will definitely have (as it
is used elsewhere in gold-plugin.cpp already).

llvm-svn: 327509