[cfi] Build __cfi_check as Thumb when applicable.

Summary:
Cross-DSO CFI needs all __cfi_check exports to use the same encoding
(ARM vs Thumb).

Reviewers: pcc

Subscribers: aemerson, srhines, kristof.beyls, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D37243

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312052 91177308-0d34-0410-b5e6-96231b3b80d8