Force an unknown target for the default case to make it work.

Summary:
This test was failing if LLVM_DEFAULT_TARGET_TRIPLE=x86_64-scei-ps4 due to
its behavior for exceptions.
Force an --unknown triple for the default case.

Reviewers: echristo, chandlerc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7422

llvm-svn: 228337