AArch64: add atomic support parameters to TargetInfo

This allows Clang to detect and deal wih __atomic_* operations properly on
AArch64. Previously we produced an error when encountering them at high
optimisation levels.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175438 91177308-0d34-0410-b5e6-96231b3b80d8