[cfi-verify] Abort on unsupported targets

As suggested in the review for r337007, this makes cfi-verify abort on unsupported targets instead of producing incorrect results.  It also updates the design document to reflect this.

Differential Revision: https://reviews.llvm.org/D49304

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337181 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed