clang/test/SemaCXX/cxx11-gnu-attrs.cpp: Add explicit -triple x86_64-unknown-unknown, or it doesn't work for targetting win32.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172404 91177308-0d34-0410-b5e6-96231b3b80d8