Do not use MEMBARRIER_MCR for any Thumb code.

Do not use MEMBARRIER_MCR for any Thumb code.
It is only supported for ARM code.  Normally Thumb2 code would use DMB instead,
but depending on how the compiler is invoked (e.g., -mattr=-db) that might be
disabled.  This prevents a "cannot select MEMBARRIER_MCR" error in that
situation.  Radar 8644195

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118642 91177308-0d34-0410-b5e6-96231b3b80d8