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.

Requires LLVM r213247.

Differential Revision: http://reviews.llvm.org/D4521