[ARM64][MC] Set the default CPU to cyclone when initilizating the MC layer.
This matches that ARM64Subtarget does for now.

This is related to <rdar://problem/16573920>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206211 91177308-0d34-0410-b5e6-96231b3b80d8