Port memory barriers intrinsics to AArch64

Memory barrier __builtin_arm_[dmb, dsb, isb] intrinsics are required to
implement their corresponding ACLE and MSVC intrinsics.

This patch ports ARM dmb, dsb, isb intrinsic to AArch64.

Differential Revision: http://reviews.llvm.org/D4520


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213247 91177308-0d34-0410-b5e6-96231b3b80d8