Implement volatile ck_cc_memcpy

[sbahra]

This is primarily needed for ck_ring. On TSO / PSO, we can fall back to builtin / library implementation, otherwise, TSO architectures require regular store implementation to avoid additional synchronization on enqueue operation. Right now we rely on silent contract of sfence for larger block sizes but some folks do not guarantee this. In addition to this, https://twitter.com/pkhuong/status/553224121359007744

ck_cc_memcpy provides ordering guarantees with respect to ck_pr_fence.