ARM: add intrinsics for the v8 ldaex/stlex

We've already got versions without the barriers, so this just adds IR-level
support for generating the new v8 ones.

rdar://problem/16227836

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204813 91177308-0d34-0410-b5e6-96231b3b80d8