Support relaxed atomic add for linux-aarch64

Atomic::add and its variants are sometimes used in HotSpot C++ code with memory_order_relaxed, in places where an atomic RMW is needed but ordering is not. The linux-aarch64 platform currently ignores the memory order argument and always performs a conservative operation, possibly affecting performance. It should instead use a relaxed atomic add where requested. Other ordering variants for add operations aren't currently used, so don't need specialization.