ARM: add support for v8 ldaex/stlex builtins.

ARMv8 adds (to both AArch32 and AArch64) acquiring and releasing
variants of the exclusive operations, in line with the C++11 memory
model.

This adds support for two new intrinsics to expose them to C & C++
developers directly: __builtin_arm_ldaex and __builtin_arm_stlex, in
direct analogy with the versions with no implicit barrier.

rdar://problem/15885451

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212175 91177308-0d34-0410-b5e6-96231b3b80d8