ck_pr: ck_pr_fence_X_Y interface has been added.

ck_pr_fence_{load_load,store_store,load_store,store_load} operations
have been added. In addition to this, it is no longer the responsibility
of architecture ports to determine when to emit a specific fence. Instead,
the underlying port will always emit the necessary instructions to
enforce strict ordering. The higher-level include/ck_pr implementation will
enforce whether or not a fence is necessary to be emitted according to
the memory model specified by ck_md (CK_MD_{TSO,RMO,PSO}).

In other words, only ck_pr_fence_strict_* is implemented by the MD-ck_pr
port.