Do not link clang_rt.cfi on Android.

Summary:
The OS provides cross-dso CFI support starting with Android O.
Trapping mode does not require any runtime at all, and diagnostic mode
requires just ubsan-standalone.

Reviewers: pcc

Subscribers: srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D38908

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315921 91177308-0d34-0410-b5e6-96231b3b80d8