[sanitizer] Do not break the CFI unwind chain in clone().

Fixes build on clang 3.2, which doesn't support cfi_undefined. This change
doesn't seem to break the slow unwinder. Even if it does, the worst thing that
could happen is that we would not see a backtrace when a fatal error happens
under StopTheWorld.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@192196 91177308-0d34-0410-b5e6-96231b3b80d8