In addition to existing Acquire/Release fences, this commit adds
SEQ_CST fence support to GHC, allowing Cmm code to explicitly emit a
fence that enforces total memory ordering. The following logic is
added:

- The MO_SeqCstFence callish MachOp
- The %prim fence_seq_cst() Cmm syntax and the SEQ_CST_FENCE macro in Cmm.h
- MO_SeqCstFence lowering logic in every single GHC codegen backend