[$] Memory-mapped I/O without mysterious macros

Concurrency is hard even when the hardware's behavior is entirely
deterministic; it gets harder in situations where operations can be
reordered in seemingly random ways. In these cases, developers tend to
reach for barriers as a way of enforcing ordering, but explicit barriers
are tricky to use and are often not the best way to think about the
problem. It is thus common to see explicit barriers removed as code
matures. That now seems to be happening with an especially obscure
type of barrier used with memory-mapped I/O (MMIO) operations.