[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

llvm-svn: 312052