Don't actually generate code for testing the frontend's target cpu flag,
just verify. This should fix the bots where the x86 backend isn't built
into Clang. Sorry for the breakage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223779 91177308-0d34-0410-b5e6-96231b3b80d8