ARM64: add acquire/release versions of the existing atomic intrinsics.

These will be needed to support IR-level lowering of atomic
operations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206489 91177308-0d34-0410-b5e6-96231b3b80d8
ARM64: switch to IR-based atomic operations.

Goodbye code!

(Game: spot the bug fixed by the change).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206490 91177308-0d34-0410-b5e6-96231b3b80d8