[x86] Add a missing triple, without which the CPU won't parse.

Notably, this is failing on our PPC build bots:
http://lab.llvm.org:8011/builders/clang-ppc64le-linux/builds/8338/steps/ninja%20check%201/logs/FAIL%3A%20LLVM%3A%3Apr33772.ll

llvm-svn: 308272