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

llvm-svn: 118642